I'm kind of jealous of all you people who can stick so closely to one or two niches. I'm all over the place. I just have so many different desires and kinks and storylines. I think it would drive me crazy trying to have a different name for every story type, personally. If I just wrote straight/gay or something like that, I think that'd be best, but I think I'd really limit myself if I did the same (i.e. one name for fantasy erotica, one for super kinky erotica, one for romance...)
You don't necessarily need pen names for each genre. I write a little of everything. Contemporary, historical, BDSM, sci-fi, fantasy, paranormal, menage, military, etc etc etc. Gay, trans, hetero, lesbian, bisexual.
Everything. I put all of my male/male under the name L. A. Witt, and everything else under Lauren Gallagher, though I've got trans fic under either name.
I'm late to this thread, but good question. I've pondered this one too. I publish m/m, f/f and m/f fiction, and I've wondered a few times if I should be using a different name for one of them. I haven't because I worry about having time to market as each of them.
It IS a bit of a pain in the ass to market multiple names. My solution was to have separate names, but keep them connected. I don't make any big secret out of the fact that L.A. Witt and Lauren Gallagher are the same person (though it has become sort of a running joke that my pen names have an ongoing rivalry). They share a website and a Twitter account, and each have their own Facebook fan page. This cuts down on some of the marketing, but I still have to work to get each name visible to her respective readers. There's some crossover among the readers, but not a lot. L.A.'s books out-sell Lauren's by a *significant* margin, so I have to work a lot harder to promote Lauren's books.
So yes, it's a good idea because M/M, M/F, and F/F are very different markets, but there is extra work involved. That shouldn't necessarily deter you, it's just something to be aware of.