Girls who Like Boys who Like Boys
The main difference, as far as I can tell, is that men often get to experience sexual interactions between women visually and openly, both in porn and not infrequently in real life at bars and parties, while women have always been more likely to turn to print to satisfy their desire for hot man-on-man action and to express this desire privately (though this is likely changing thanks to the mainstreaming of gay porn and the aforementioned increase in physical affection between men). Part of this has to do with cultural norms about sexual behavior. We often view women's sexuality in terms of performance for the sake of men, so it follows that some women would willingly act out Sapphic scenarios if they realize that men find them appealing. The fact that men also traditionally feel more comfortable expressing their sexual desires also plays into such behavior. It's not uncommon to see men egging women on to make out with each other, but it's rare you'll see women doing the same to men. Bi- and homosexuality are also more acceptable in our culture for women than for men, so women may feel more comfortable publicly displaying their erotic attraction to each other. They're also likely to get a more positive reaction from onlookers -- if I drunkenly make-out with a guy, chances are people will yell at us to get a room, but if I do so with another woman then my biggest problem is usually shoving away audience members who want to get a little too close. (For the record, I don't make out with women to get male attention, it's just that I will make out with almost anyone if I'm drunk and they nuzzle my neck in the right way. It's like there's a literal "turn on" button located in my neck.) Two men making out in public, on the other hand, can expect a more negative reaction than even a heterosexual couple, mostly because other men are more vocal about their disgust at such a sight than women are about their arousal.
Women may not be as likely as men to come right out and ask other people to make out with each other, but that doesn't mean they don't want to see (or at least imagine) two men together. There is a lot of material out there to fill that desire, although most people aren't aware of it, or at least don't realize how widespread it is. Slash fiction, for instance, a type of fan fiction that focuses on sexual relationships between male characters from movies, TV, and books, is primarily written and read by heterosexual women. This isn't a new phenomenon, either, although the internet has broadened the audience greatly. The earliest examples of such fiction were written in the 70s and were based on Star Trek (Kirk and Spock are the original Slash couple). Another common form of man-love fiction which originated in the 70s and whose primary audience is straight women is Yaoi, a Japanese term for fiction (usually manga, and usually sexually explicit) involving homoerotic relationships between men. Male/male romance novels, a rapidly growing subgenre, are also primarily written and read by women. (And lest you think that the appeal here is the romance, let me assure you that almost nobody reads cheap romance novels for anything but the graphic descriptions of sex. Those books are girl-porn.) Anecdotally, a lot of straight women also like watching gay porn, and even prefer it to heterosexual porn, although actual statistics on this are difficult to find.
The reasons women give for enjoying this kind of fiction are diverse. Imagining two men together conveniently gets around the political and social pressures women deal with in regards to their own sexuality and lets them just enjoy the action. Additionally, some women want to experience something dramatically different from their real sex lives, and gay sex is about as different as it gets. Probably the biggest reason of the appeal of man-love fiction and videos, however, is that for heterosexual women, it provides two times the sexy. When you get down to the basics, pornographic works (both visual and written) operate primarily on two levels - the empathetic and the attractive. Porn can be hot because imagining yourself in the position of one of the characters gives you a vicarious thrill, or it can be hot because you are sexually attracted to one (or more) of the characters. If the latter is what you're most interested in, then male-on-male erotica has the same advantage for straight women that lesbian erotica has for straight men - it's twice the number of the gender you're attracted to with no distracting members of the gender you aren't. Of course women find that hot.
This isn't to say that I think men should go around making out with each other just to arouse women. I'd prefer, frankly, that no one perform any sexual act solely for the titillation of others. But if men want to be able to enjoy watching women make out with each other, then it's only fair to return the favor once in a while. As many of the ladies here will happily tell you, it would be greatly appreciated.
Dr. Pisaster has a doctorate in biophysics, not actually anything sexy. She does however enjoy having sex, reading about sex, and talking about sex. Especially when she's had a little whiskey.