I think it's different from guys watching lesbian porn. When it comes down to it male homosexuality is seen as disgusting by our society compared to female homosexuality, not only superficially but also when people consider the actual sex acts involved themselves. Women aren't immune to this societal standard (be it double or... whatever), and I think many of them think of male homosexual acts as just as disgusting as heterosexual males think them to be. At any rate I think there is a different dynamic here.