I don't know. I just can't keep my dick in my pants. I see some hot ass and I just go for it. And somehow the fact that I'm cheating makes it even more exciting.
Seriously. Sexual role preferences are not sexual orientations. You are not born top or bottom. You choose to be top or bottom based on your own likes and dislikes. If you really feel that being a bottom makes you ashamed of yourself, maybe work on that through therapy. But if you really can't deal with being a bottom, remember there is a top shortage. The solution to top shortage is between your legs.