I'm not sure it is a stereotype, but I think a lot of people are under the misconception that when an adult male abuses young boys, that makes him a homosexual. This is used as another example of what makes homosexuality a bad thing. As people have already pointed out, statistically it is more likely the man will be heterosexual, but because of the prejudice that goes along with homosexuality, you have to be careful. I don't think it is a problem to have a homosexual predator, but it must be done carefully and with the knowledge that this doesn't mean that ALL homosexuals are predators. When a category of people is stereotyped, and prejudiced in one way, and then someone writes something to that stereotype, even if they are doing it with their eyes wide open, it makes the writing much more tricky in order to convince your readers that you have made a specific choice and not just falling into some prejudicial trap (ie greedy Jews, or an african american who is a criminal).
Also the term "homosexual predator" can cover any number of things, from a homosexual man who preys on men his own age etc. A pedophile has a specific target, one which the OP seems to want to address.
Not saying it can't be done, but man, it has to be done well.