Gay men are known to be more exposed to HIV infections than heterosexual people are. But a new research has discovered that the significant differences regarding the HIV infection in the US between gay men and straight men and women are determined by factors that go far beyond sexuality.

In 2005, more than 50 % of the new HIV infections in the US were among gay men, and up to 20 % of the urban American gay population is HIV positive. Still, two large population surveys revealed an average number of annual unprotected sexual partners for gay men similar to that of straight men and women.

The researchers used various mathematical methods to assess the speed at which the HIV infection has spread among gay men and straight men and women. The data delivered by the two national surveys was employed to assess how many sex partners gay men and straight men and women have, and the percentages of gay men having insertive or receptive anal sex, or both.

These numbers were employed for assessing how easily HIV is transmitted by vaginal and anal sex to determine the size of the HIV epidemic in homosexual men and heterosexual men and women. The mathematical calculations revealed that for the heterosexual US population to experience an epidemic of HIV infection at the levels achieved by the gay men population, they would have to have about five unprotected sexual partners annually, about 3 times more than the gay men do.

Instead, to stop the HIV epidemic, gay men would have to have unprotected sex several times less than those currently determined among the heterosexual population, as HIV is more likely to be transmitted through anal sex than through vaginal sex.

But "role versatility", when a gay man has both "insertive" and "receptive roles," also increases the risk. A gay man can easily catch the HIV through unprotected receptive sex, and then transmit it to someone else through insertive sex. This explains the much higher number of HIV cases amongst the gay population, even if they have the same numbers of unprotected sexual partners as heterosexual people have.