HIV can be passed through the mucous membranes (all the deep pink stuff that isn't skin). It is not necessary to have a broken or damaged membrane in the uninfected individual for this to occur; it is probably mostly dependent on the viral load in the infected individual. As some people are almost asymptomatic, this is not something that can easily be predicted.
Prevalence is one thing, but gay HIV carriers are also far more likely to be tested, begin taking retrovirals (which can all but negate the risk of infecting a partner when used in conjunction with barrier prophylactics) and avoid sex with uninfected individuals once they are themselves diagnosed. By contrast, heterosexual men cause a statistically significant higher number of infections per individual, regardless of whether they have or have not been diagnosed.