My theory is that gay men have lots of sex with as many different partners because they can. Straight men would do the same, if women were willing. AFAK, lesbians aren't any more promiscuous than straight women because, in general, women do not have the same biological drive to have sex with anything that moves.
You are wrong. Lesbians are more promiscuous than normal women. They are also more prone to beating each other up, various kinds of STDs, and alcohol and drug abuse.