Men are more promiscuous than women. Remove women from the equation, and it naturally follows that gay men (or 'straight' men who engage in M/M sex) would be more promiscuous than their 100% straight brothers. There are a lot of reasons for this. Men are opportunistic and ready to go at the drop of a hat (or a zipper), whereas women typically require some seduction or romance. Historically the gender roles in society have allowed men more freedom and opportunities for sex. It's easier for a man to hide his sexual liasons, where it's logistically more complicated for a woman to avoid discovery - trysting in an out-of-the-way, out-of-sight place, fixing her clothes, hair, makeup, etc. afterward. Men don't generally worry about being considered a 'slut', on the contrary, the more sex a man has, the 'bigger man' he is. In addition, gay men don't typically have the same sort of family commitments and expectations of monogamy put on them that straight men do.
Yes, gay (or 'bi') men have more sex, and I think it's a source of resentment for many straight men, and for Puritanical religious fundamentalists of both genders in particular. Subconsciously at least, I think they're jealous, and that's what accounts for a lot of anti-gay prejudice. imho.