I chalk that up to 2 things. 1. Men may or may not admit it but they will fantasize about sex with two women. It appeals to them however sex with men disgusts them. 2. Men are the ones making the most noise about this and as such, since they loathe male homosexuality much more, hence the focus.
It's not just an American thing, either. It's widespread in various cultures. Being penetrated by another man or male is usually viewed as "lowering" oneself to the status of a woman or being "emasculated". It's a very ancient notion and was prevalent throughout the Hellenic world, especially. In the Hellenic world, the one who penetrates wasn't really stigmatized, because to penetrate, in of itself, was viewed as masculine. So for an adult man to take on a young male lover or a male who is viewed as feminine and penetrate him wasn't viewed as a problem. Those ideas spread throughout the Mediterranean, the Middle East, North Africa, etc. which all tend to have "macho" cultures. They also have very sizable homosexual "undergrounds" and the practice of pederasty is common.
Basically, it's a "macho" patriarchal view and it's tied into sexism. Men raised in those cultures tend to be pretty neurotic when it comes to that, because it's so culturally tied into their notions of what masculinity is, itself. They don't want to be seen as "sissies", "punks" and "b_tches". Look at prison, for example. Male prisons are hypermasculine environments and male on male rape is used as a way of humiliating and dominating another male, and that's because our culture tends to see taking the penetrative role in sex as inherently being dominating and an act of possession.
Anyway, that view of sexuality and imbuing genitals with some sort of binary categorization or function has basically made our culture pretty bonkers about sex. Sexism, homophobia and transphobia are all strongly interlinked and they're mostly the product of male neurosis with leads them to suppress and hurt themselves and also to oppress and hurt others. Men in general need to do a lot of self-searching and grow the hell up, in general.
Women's sexuality has been much more ignored, historically and socially. They really didn't care what women did, as long as the women the men wanted were sexually available to them. Of course, the typical straight man's fantasy of women being sexual with each other is male-centric and heterosexist. It erases and objectifies lesbians and their actual sexuality. Because they obviously just need to be banged well enough by a guy in order to "fix" them and make them see what they're "missing", and obviously lesbianism exists to serve male needs.
