The human body is not designed to accommodate anal intercourse.
Whereas the vagina has natural lubricants and is composed of a mucus membrane with a multi-layer stratified squamous epithelium that allows it to endure friction without damage and to resist the immunological actions caused by semen and sperm, the anus is a delicate mechanism of small muscles that comprise an “exit-only” passage. With repeated trauma, friction and stretching, the sphincter loses its tone and its ability to maintain a tight seal.
Consequently, anal intercourse leads to leakage of fecal material that can easily become chronic. Moreover, the intestine has only a single layer of cells separating it from blood. Therefore, any organisms that are introduced into the rectum have a much easier time establishing a foothold for infection than they would in a vagina. The single layer tissue cannot withstand the friction associated with penile penetration, resulting in traumas that expose both participants to blood, organisms in feces, and a mixing of bodily fluids.
Furthermore, ejaculate has components that are immunosuppressive, designed to allow the sperm to evade the immune defenses of the female. The fragility of the anus and rectum, along with the immunosuppressive effect of ejaculate, make anal-genital intercourse a most efficient manner of transmitting HIV and a whole host of other infections. [For a list of the diseases, go here.]
Despite the health hazards of anal sex, Reuters reports, March 3, 2011, that the U.S. Centers for Disease Control and Prevention (CDC) said in 2006-2008, 21% of U.S. young men and 20% of young women said they have had anal sex.
The report did find that the number of U.S. teenagers and young adults who say they are abstaining from sex has slightly increased. Among Americans ages 15 to 24, 29% of women and 27% of men reported not having any sexual contact compared with 22% in 2002.
At the same time, however, more young women report having sexual contact with each other. The CDC’s National Center for Health Statistics found a growing number of young women who said they have had some form of sex with another woman, rising to 13% in the latest survey, from about 12% in the 2002 survey. In contrast to the number of young men reporting same-sex encounters falling from 5% to 4% over the same time period.
Of youth aged 15 to 24 who said they have had sex, nearly 63% of women and 64% of men had oral sex compared to nearly 69 percent in 2002.
The CDC’s findings for 2006 to 2008 are based on interviews with about 13,500 men and women ages 15 to 44. Overall, it found few changes in the nation’s sexual patterns compared to CDC’s last survey in 2002.