There are two issues here.
First, unprotected anal sex is for obvious reasons the most dangeours kind of sex for HIV spreading. That applies to both male/male and male/female anal sex. Male homosexuals are, of course, more likely to have anal sex than either heterosexuals or lesbians (which naturally don't have such sex at all). Second, male homosexuals are far more likely to have many more partners for sex than heterosexual women (or men, for that matter). Put the two together, and you get a rather obvious reason why male homosexuals are far more at risk for HIV than other groups.
First, unprotected anal sex is for obvious reasons the most dangeours kind of sex for HIV spreading. That applies to both male/male and male/female anal sex. Male homosexuals are, of course, more likely to have anal sex than either heterosexuals or lesbians (which naturally don't have such sex at all). Second, male homosexuals are far more likely to have many more partners for sex than heterosexual women (or men, for that matter). Put the two together, and you get a rather obvious reason why male homosexuals are far more at risk for HIV than other groups.