However, the sexual behaviors of MSM and of male and female heterosexuals are substantially different in ways that are not explained by biology alone. These factors alone would result in significant disparities in HIV rates between MSM and heterosexuals even if both populations had similar numbers of sex partners, frequency of sex, and condom use levels 8. Furthermore, the transmission probability of HIV associated with anal sex is higher than that associated with vaginal sex 6, 7. demonstrated that an MSM population with a very high level of versatility would have a higher HIV prevalence than one with less versatility 5. In part, these differences reflect the fact that an individual MSM can engage in both insertive and receptive sexual roles (i.e., versatility), while exclusively heterosexual men and women each engage in only one of these roles. The Centers for Disease Control and Prevention (CDC) estimates that HIV and early syphilis rates among MSM are >40 times higher than those among heterosexuals 4. population 1, but accounted for 59% of new HIV infections and 62% of cases of early syphilis in 2009 2, 3. MSM comprise approximately 2% of the U.S. Sexually transmitted infections (STI) disproportionately affect men who have sex with men (MSM).
