To be honest, the only thing that stopped me from doing it when I was younger was like Maz said, I'm not exactly a beauty queen, and also...errm I have never really been very good at being a bottom My botbot doesn't like having things in it so much, which is fine when you're just having a reg'lar sex life because you know, there are plenty of gay folk in the world who love all sorts of different things blah de blah, but I imagine it would limit yourself a bit being a man-for-hire (and yeah I know there is big demand for sub/dom roleplay where very little to no actual penetrative sex comes into it, but I never really had the confidence until comparitively recently to do that kind of stuff even remotely and even now I'm not very GOOD at it most of the time).
The only thing that would stop me now I guess is that I would be concerned about passing on HIV to someone if a condom broke etc....which is strange because it doesn't limit my normal sex life at all, but I guess there's an extra level of responsibility there in my head if it's something I'm doing for a job.
Oh, and also I'm just not really that interested in being a hooker. I love sex crazylike, but any interest I ever had in working in the sex industry is long gone...
To answer your questions Maz (and make this post even more long and boring than it already is), I would only have sex with guys and keeping it up in a lot of ways I think is a confidence thing; I used to have more issues with it when I was younger and had very low self-esteem than I ever have in the past few years.