I have to agree with previous posters.. choose a subject that interests you. If the only thing that interests you is sex then find another interest. You're on a webmasters forum right now so that must interest you a bit.. why not start one too? Just choose something that won't bore you and you can post quality content about. You shouldn't start a forum because of the money, start one because you're passionate about a certain subject and enjoy discussing it
you should start a forum on something you like and have acknowledge on, do not make things just for the money, if you are doing what you know, money is a consequence