Male vs. Female Homosexuality
I think the reason Lesbians are more acceptable than Gay men is because girls are naturally much closer to their friends of the same sex, and seeing to girls hugging doesn't suggest they are Lesbians, wheras men tend to have a much more distant relationship with their male firends. Also, many people find the idea of $%!@ sex pretty disgusting (in both gay and straight couples) wheras Lesbian sex is far more acceptable. It is also the case that most girls find the idea of two men discusting, whereas men find two girls a turn on.
duh. men wants sex with the two girls
I'm not sure what that has to do with it. Somehow, I doubt it is too terribly often that two lesbians are going to be interested in having sex with a man who is just lusting for the opportunity to have two girls. I know I've never experienced that. My girlfriend and I have never invited a man into our bedroom for sex. Other women? Well, that's a different question altogether now, isn't it? ;-)
I think Disoblige is right. It's a little known male fantasy, that's why it's more accepted, in an androcentric society.

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