What boobinator said.
Also, keep in mind that it's the company who decided who does who (unless the stars have their own company/site) and if a company likes a certain girl and wants them to do a lesbian scene & the star doesn't object (I really doubt Ava vincent would have done Dillan Lauren, no matter how much she got paid) it happens.
I don't know how many companies there are, but even if a majority only produces b/g or m/f scenes/shoots, there are still enough companies that will ask the girls to do g/g or f/f scenes/shoots and I have to say I doubt most companies only produce b/g-m/f scenes.
This means that every girl in the bussiness will be asked to do a g/g-f/f or at least a m/f/f-b/g/g scene/shoot at least once (& probably more than once).
Also don't forget that a lot of girls don't start out doing b/g-m/f hardcore. They start doing solo softcore movies/shoots, either with or without toys. The next step if other girls and then they start doing other guys.
There are also girls that won't do men when they have a boyfriend unless it is their boyfriend. I'm not entirely sure (& I can't really prove it), but I believe Sophie Moone was one of those girls (it would explain the limited amount of b/g scenes/shoots she has done).