Beyond the inherent sexual appeal of having your preferred genders genitalia in your face, there is a psychological aspect of dominance and submissiveness. Men who actively enjoy sucking dick tend to be submissive, and just the idea of sucking a dick turns them on because it makes them feel 'used'. Evolution wise some scientists believe that this is because gay men served as underlings to the alpha of a pack. They'd still have male based duties in the pack, but would be subservient to him sexually and socially.
Others assume its just because gay men are more feminine and therefore think like girls who would become submissive. But its neither here nor there.
The point is that for a lot of gays (not all, maybe not even most, but a lot) are very submissive. It turns them on the same way that grabbing a girl by the wrists and holding her down turns you on. Its the inverse of that. You like to feel in charge and dominating. They like to feel dominated.