1. Evidence shows there are precious few truly bi folks, and that the brunt of bi folks are lying they prefer one to the other but lack the courage to say so.
2. The nature of some disorders is to occur in tandam with others. Homosexuality has already been proven to be frequently co-morbid with depression. It is extremely likely that if one is homosexual they also have other sexual fetishes of which same sex intercourse is but one.
Once you start playing outside the sandbox, you're prone to play in any ol dirt you find yourself in. (Social psychology studies show that as well)