Even though anal intercourse is common among both heterosexual and homosexual couples, little is known about how this practice might impact bowel function, Markland and colleagues note in the American Journal of Gastroenterology. All participants underwent pelvic MRI in the supine position. An Encyclopedia states that "the inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and that "the rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand". The IAS plays a key role in maintaining continence, providing 75 percent of the maximum resting pressure. What data we do have almost all relate to penetrative sexual acts, and the superficial contact of the anal ring with fingers or the tongue is even less well documented but may be assumed to be a common sexual activity for men of all sexual orientations. This may be because of the taboo nature of anal sex and that teenagers and caregivers subsequently avoid talking to one another about the topic. It has been, and in some jurisdictions continues to be, a crime carrying severe punishment. In pornography, anal sex is commonly portrayed as a desirable, painless routine that does not require personal lubricant ; 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 discomfort or pain instead of pleasure from the activity. Measurements Axial MR images were imported into an imaging program three-dimensional Slicer, version 2. However, to determine these relationships, the radius of the tubular structure must be known. Cosmetics Shaving, trimming, depilatory hair removal , or Brazilian waxing can clear the perineum of hair. Because of this, some couples practice anal intercourse as a form of contraception , often in the absence of a condom. Because of the view that anal sex is not "real sex" and therefore does not result in virginity loss, or pregnancy, teenagers and other young people may consider vaginal intercourse riskier than anal intercourse and believe that a STI can only result from vaginal intercourse. The latter was allowed because anal intercourse was considered equivalent to vaginal intercourse in this way; men were said to "take it like a woman" muliebria pati, "to undergo womanly things" when they were anally penetrated, but when a man performed anal sex on a woman, she was thought of as playing the boy's role. The review suggests that anal sex is exotic among the sexual practices of some heterosexuals and that "for a certain number of heterosexuals, anal intercourse is pleasurable, exciting, and perhaps considered more intimate than vaginal sex". Inner and outer diameters were measured from axial magnetic resonance images and used to calculate sphincter thickness and mean radius by two independent investigators blinded to patient age.