Another pervert checking in here (at least among the female and straight variety) and, although I’m perfectly fine with lesbian action, I can’t say it does much for me one way or another. However, if we’re talking it’s of two guys I find hot or better, The Hottest Guy in The World (to me) cloned, oh dear Og, where do I sign up? I’ll stop panting right about now. 
Now you’re just being redundant.
For the OP, I think that it’s simpler than some people have been making it out to be: Many people like to see two members of their preferred gender making out, because it’s a fantasy. And the fantasy, in their mind, is that if they were suddenly transported into the scene, they’d become the focus of the attention of the people in the scene.
IOW, it’s not about the lesbians, or the gay men - it’s about what sort of fantasies the viewer will build off those images, regardless of the reality of the preferences of the people in question.
And now, reading a bit further into the thread, I see that Miller has shot down that theory. Curse you, Miller, and your facts getting in the way of my perfectly good theory! 
Just to confirm what you’ve said: I’ve written over a hundred homoerotic fanfic stories myself, and I know hundreds of other women who write slash–that’s how I met most of my friends. All of us apparently pervs (or non-existant; I’m surprised and amused by the number of men who insist that women can’t possibly be interested in male/male erotica and refuse to believe it even when told otherwise.) Oh, and I know several bi and lesbian women who write or enjoy man-on-man action.
I’ve never personally been interested in porn films or images, homo or hetero. I tend to find it boring and impersonal–more about anonymous body parts than people. The homoerotic films I have are much softer, mainstream stuff that involves characters with an emotional interaction–Maurice, Brokeback Mt, My Beautiftul Launderette. And of course a good slash writer can dig up homoerotic subtext out of practically anything involving two (or more) guys even when they have their clothes on.
I’ll hold my hand up,yes I do find lesbians an incredible turn on …er I’ll get my coat.
I agree that it’s mostly just seeing two women aroused and in sexual activity without also having to see a guy.
But, I’ve also found that some straight porn feels the need to somehow degrade the women involved. Like fucking some over-muscled dude is an awful crime, and her punisment is to fuck another over-muscled dude. Stuff that’s not necessary outright S&M or fake rape, but has the same general theme of “you will be DOMINATED by my cock!” This is mixed in freely with the porn that’s more simply about a guy and girl having some good dirty fun, and I can’t be sure which I’m getting when I open a clip.
While I’m sure they exist, I’ve never seen a lesbian video that wasn’t just two chicks having fun with each other. Degrading lesbian sex (if we first allow that having sex with another woman on camera isn’t itself degrading) is either much rarer, or better labled as a specialty niche. I prefer girl-girl action because I’m a feminist.
Also, semen isn’t a turn on for me. Not in the self-loathing “I’m ejaculating evil!” sense, but it’s just another bodily fluid I’d rather not see. And straight porn often fixates on it. This is obviously not a problem for lesbian porn.
The problem I have with porn that involves males is that it quickly veers off into blow jobs. Blow jobs are not sexy to me. I have no interest in seeing the apparently lobotomized face of some female with her mouth full of dick. (Why do they all look so totally devoid of personality?)
Couldn’t they show her groping him, playing with him? There’s not even enough of him playing with her. Usually a bit of that and then straight to the freaking blow job.