I would say that if you respond sexually to the sight of a cock or a man's ass (or his chest or his thighs or his size) that would make you very definitely "bi" (I'd say 50% gay - in the lexicon here - at the very least). If you were less, it would take you forever to get aroused...unless, of course, the "forbidden," "dirty," or "the road less traveled" is mainly what stokes your banked fires (and that is a factor sometimes - hey, I'm minimally into gals, but should an attractive nun find me appealing...well, that would stoke my fire quite a lot!)
And, of course, the very young can sometimes get off just by the thought of fucking a lamppost!