'Gay' is strict homosexual behavior. Homosexuality in animals and primate relatives always occurs alongside heterosexual behavior, because homosexual behavior is a great pair bonding mechanism that increases fitness, but is ultimately worth very little if you don't reproduce.
'Gay' is an exacerbation of homosexual behavior that most/all humans are capable of in the right circumstances, but only a few become susceptible to in the face of prevailing cultural demonization of the behavior for a variety of reasons and predispositions, some of which are genetic but sparingly few of which are determinative.