I'm not a psychologist, a behaviourist nor a sexual analyst, so this is definitely something of a pet theory. But I've always considered this issue with regards to the respective roles that 'masculinity' and 'femininity' play.
Somewhere, almost certainly as a product of evolution, it's hardwired into us that men are meant to act in a 'masculine' way: i.e. hunter/killer/protector/provider. You know the sort of thing. And for some people (male **and** female), the sight – or even the idea – of two men "getting it on" flies in the face of that hardwired, traditional concept of 'manliness'. I also think that some men, confronted by their own desire for sex with other men, may find that desire to be threatening to their own sense of masculinity, and therefore threatening to their very identity.
For some reason though, the same set of conflicting thoughts and emotions are not triggered by the sight or thought of two women "getting it on". And I think it has something to do with the way that such an act does not fly in the face of any hardwired concept we have about the female role. Female-female sex does not threaten a woman's femininity. In fact, it may even enhance it. Why that should be is a question for someone with a far more profound appreciation of this field than I.
Now before anyone jumps on me from a great height, I am **not** repeat **not** suggesting that a man who enjoys sex with another man (either as part of a homosexual or bisexual relationship) is not masculine, or has surrendered his masculinity. Nor am I suggesting that all men with sexual desires for other men feel that their masculinity or identities are threatened. This is a pet theory about the role that hundreds of thousands of years of biological programming may play in the question of why male-male sex still remains something of a taboo even among a crowd as sexually enlightened as swingers, while female-female sex is positively eulogised about.
Even if this theory has a degree of truth to it, I suspect it's just one element of what is an incredibly complex issue.