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Anal sex or anal intercourse is generally the insertion and thrusting of the erect penis into a person's anus, or anus and rectum, for sexual pleasure. Other forms of anal sex include fingering, the use of sex toys for anal penetration, oral sex performed on the anus (anilingus), and pegging. Though the term anal sex most commonly means penile-anal penetration, sources sometimes use the term anal intercourse to refer exclusively to penile-anal penetration, and anal sex to refer to any form of anal sexual activity, especially between pairings as opposed to anal masturbation.

While anal sex is commonly associated with male homosexuality, research shows that not all gay males engage in anal sex and that it is not uncommon in heterosexual relationships. Types of anal sex can also be a part of lesbian sexual practices. People may experience pleasure from anal sex by stimulation of the anal nerve endings, and orgasm may be achieved through anal penetration – by indirect stimulation of the prostate in men, indirect clitoral/G-spot stimulation in women, and associated sensory nerves (especially the pudendal nerve). However, people may also find anal sex painful, sometimes extremely so, which may be primarily due to psychological factors in some cases.

As with most forms of sexual activity, anal sex participants risk contracting sexually transmitted infections (STIs/STDs). Anal sex is considered a high-risk sexual practice because of the vulnerability of the anus and rectum. The anal and rectal tissues are delicate and do not provide natural lubrication, so they can easily tear and permit disease transmission, especially if lubricant is not used. Anal sex without protection of a condom is considered the riskiest form of sexual activity, and therefore health authorities such as the World Health Organization (WHO) recommend safe sex practices for anal sex.

Often, strong views are expressed with regard to anal sex; it is controversial in various cultures, especially with regard to religion, commonly due to prohibitions against anal sex among gay men or teachings about the procreative purpose of sexual activity. It may be regarded as taboo or unnatural, and is a criminal offense in some countries, punishable by corporal or capital punishment; by contrast, people also regard anal sex as a natural and valid form of sexual activity that may be as equally fulfilling as any other desired sexual expression. They may regard it as an enhancing element of their sex lives or as their primary form of sexual activity.

Anatomy and stimulation

The abundance of nerve endings in the anal region and rectum can make anal sex pleasurable for men or women. The internal and external sphincter muscles control the opening and closing of the anus; these muscles, which are sensitive membranes made up of many nerve endings, facilitate pleasure or pain during anal sex. "The inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and "[t]he rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand".

Research indicates that anal sex occurs significantly less frequently than other sexual behaviors, but its association with dominance and submission, as well as taboo, makes it an appealing stimulus to people of all sexual orientations. In addition to sexual penetration by the penis, people may use sex toys such as butt plugs or anal beads, engage in fingering, anilingus, pegging, anal masturbation or fisting for anal sexual activity, and different sex positions may also be included. Fisting is the least practiced of the activities, with "[f]ew people" being "capable of relaxing enough to accommodate something as big as a fist in their anus, and there is real danger of damage to the delicate rectal tissues".

In a male receptive partner, being anally penetrated can produce a pleasurable sensation due to the inserted penis rubbing or brushing against the prostate (also known as the "male G-spot", "P-spot" or "A-spot") through the anal wall. This can result in pleasurable sensations and can lead to an orgasm in some cases. Prostate stimulation can produce a "deeper" orgasm, sometimes described by men as more widespread and intense, longer-lasting, and allowing for greater feelings of ecstasy than orgasm elicited by penile stimulation only. The prostate is located next to the rectum and is the larger, more developed male homologue (variation) to the female Skene's glands (which are believed to be connected to the female G-spot). However, though the experiences are different, male orgasms by penile stimulation are also centered in the prostate gland. It is also common for men to not reach orgasm as receptive partners solely from anal sex, and it may be that it is typical that receptive male partners do not reach orgasm solely by this activity.

General statistics indicate that 70–80% of women require direct clitoral stimulation to achieve orgasm. The clitoris is composed of more than the externally visible glans (head). With its glans or body as a whole estimated to have around 8,000 sensory nerve endings, the clitoris surrounds the vagina and urethra, and may have a similar connection with the anus. The vagina is flanked on each side by the clitoral crura, the internal "legs" of the clitoris, which are highly sensitive and become engorged with blood when sexually aroused. In addition to nerve endings present within the anus and rectum, women may find anal stimulation pleasurable due to indirect stimulation of these "legs". Indirect stimulation of the clitoris through anal penetration may also be caused by the shared sensory nerves; especially the pudendal nerve, which gives off the inferior anal nerves and divides into two terminal branches: the perineal nerve and the dorsal nerve of the clitoris.

The Gräfenberg spot, or G-spot, is a debated area of female anatomy, particularly among doctors and researchers, but it is typically described as being located behind the female pubic bone surrounding the urethra and accessible through the anterior wall of the vagina; it is considered to have tissue and nerves that are related to the clitoris. Besides the shared anatomy of the aforementioned sensory nerves, orgasm by stimulation of the clitoris or G-spot through anal penetration is made possible because of the close proximity between the vaginal cavity and the rectal cavity, allowing for general indirect stimulation. Achieving orgasm solely by anal stimulation is rare among women. Direct stimulation of the clitoris, G-spot, or both, during anal sex can help some women enjoy the activity and reach orgasm from it.

Stimulation from anal sex can additionally be affected by popular perception or portrayals of the activity, such as erotica or pornography. In pornography, anal sex is commonly portrayed as desirable, routine, without use of a personal lubricant or a condom, and painless; this can result in couples performing anal sex without care, and men and women believing that it is unusual for women, as receptive partners, to find no pleasure from the activity and instead discomfort or pain from it. By contrast, each person's sphincter muscles react to penetration differently, the anal sphincters have tissues that are more prone to tearing, and the anus and rectum, unlike the vagina, do not provide natural lubrication for sexual penetration. Researchers say adequate application of a personal lubricant, relaxation, and communication between sexual partners are crucial to avoid pain or damage to the anus or rectum. Ensuring that the anal area is clean and the bowel is empty, for both aesthetics and practicality, may also be desired.

Health risks

General risks

Anal sex can expose participants to two principal dangers: infections due to the high number of infectious microorganisms not found elsewhere on the body, and physical damage to the anus and rectum due to their fragility. Increased experimentation with anal sex by people without sound knowledge about risks and what safety measures do and do not work may be linked to an increase in sexually transmitted infections (STIs/STDs); for example, teenagers may consider vaginal intercourse riskier than anal intercourse and believe that a STI can only result from the former activity. Unprotected penile-anal penetration, colloquially known as barebacking, carries a higher risk of passing on STIs because the anal sphincter is a delicate, easily-torn tissue that can provide an entry for pathogens. The high concentration of white blood cells around the rectum, together with the risk of tearing and the rectum's function to absorb fluid, places those who engage in the practice at high risk of STIs. Use of condoms, ample lubrication to reduce the risk of tearing, and safer sex practices in general, reduce the risk of STI transmission. However, a condom can break or come off during anal sex, so both sex partners must remain watchful during sex and there is still some risk if one or both partners carries a sexually transmissible infection.

Unprotected receptive anal sex is the sex act most likely to result in HIV transmission. Other infections that can be transmitted by unprotected anal sex are human papillomavirus (HPV) (which can increase risk of anal cancer); typhoid fever; amoebiasis; chlamydia; cryptosporidiosis; E. coli infections; giardiasis; gonorrhea;[20] hepatitis A; hepatitis B; hepatitis C; herpes simplex; Kaposi's sarcoma-associated herpesvirus (HHV-8); lymphogranuloma venereum; Mycoplasma hominis; Mycoplasma genitalium; pubic lice; salmonellosis; shigella; syphilis; tuberculosis; and Ureaplasma urealyticum.

There are a variety of factors that make male-to-female anal intercourse riskier for a female than for a male. For example, besides the risk of HIV transmission being higher for anal intercourse than for vaginal intercourse, the risk of injury to the woman during anal intercourse is significantly higher than the risk of injury to her during vaginal intercourse because of the durability of the vaginal tissues compared to the anal tissues. Additionally, if a man moves from anal intercourse immediately to vaginal intercourse without a condom or without changing it, infections can arise in the vagina due to bacteria present within the anus; these infections can also result from switching between vaginal sex and anal sex by the use of sex toys.

Though anal sex alone does not lead to pregnancy, pregnancy can still occur with anal sex or other forms of sexual activity if the penis is near the vagina (such as intercrural sex or other genital-genital rubbing) and its sperm is deposited near the vagina's entrance and travels along the vagina's lubricating fluids; the risk of pregnancy can also occur without the penis being near the vagina because sperm may be transported to the vaginal opening by the vagina coming in contact with fingers or other non-genital body parts that have come in contact with semen.

Pain during receptive anal sex among gay men (or men who have sex with men) is formally known as anodyspareunia. One study found that about 12% of gay men find it too painful to pursue receptive anal sex, and concluded that the perception of anal sex as painful is as likely to be psychologically or emotionally based as it is to be physically based.[105] Another study that examined pain during insertive and receptive anal sex in gay men found that 3% of tops (insertive partners) and 16% of bottoms (receptive partners) reported significant pain. Factors predictive of pain during anal sex include inadequate lubrication, feeling tense or anxious, lack of stimulation, as well as lack of social ease with being gay and being closeted. Research has found that psychological factors can in fact be the primary contributors to the experience of pain during anal intercourse and that adequate communication between sexual partners can prevent it, countering the notion that pain is always inevitable during anal sex.

Physical damage and cancer

It is uncommon for serious physical injury to occur as a result of anal sex, but anal sex can exacerbate hemorrhoids and therefore result in bleeding; if bleeding occurs, it may also be a result of a tear in the anal or rectal tissues or perforation (a hole) in the colon, which is a serious medical issue that should be remedied by immediate medical attention. Repetitive penetrative anal sex may also result in the anal sphincters becoming weakened, which may affect the ability to hold in feces (a condition known as fecal incontinence); Kegel exercises have been used to strengthen the anal sphincters and overall pelvic floor, and may help prevent or remedy fecal incontinence.

A 1993 study indicated that fourteen out of a sample of forty men receiving anal intercourse experienced episodes of frequent fecal incontinence. However, a 1997 study found no difference in levels of fecal incontinence between gay men who engaged in anal sex and heterosexual men who did not, and criticized the earlier study for its inclusion of flatulence in its definition of fecal incontinence.

Most cases of anal cancer are related to infection with the human papilloma virus (HPV). Anal sex alone does not cause anal cancer; the risk of anal cancer through anal sex is attributed to HPV infection, which is often contracted through unprotected anal sex. Anal cancer is relatively rare, and significantly less common than cancer of the colon or rectum (colorectal cancer); the American Cancer Society states that it affects approximately 7,060 people (4,430 in women and 2,630 in men) and results in approximately 880 deaths (550 in women and 330 in men) in the United States, and that, though anal cancer has been on the rise for many years, it is mainly diagnosed in adults, "with the average age being in the early 60s. The disease affects women somewhat more often than men." Though anal cancer is serious, treatment for it is "often very effective" and most anal cancer patients can be cured of the disease; the American Cancer Society adds that "receptive anal intercourse also increases the risk of anal cancer in both men and women, particularly in those younger than the age of 30. Because of this, men who have sex with men have a high risk of this cancer.

(source: WiKiPedia)