Only engaging in homosexual activity in particular situations. Such situations could be a party, spring break, really, any event involving alcohol.
What a wife tells herself to make herself feel better. "My husband only makes out with guys because he's 'situationally gay', he's not 'really gay'.
Having a gay relationship with the situation or opportunity presents itself or appears attractive. The person is not necessarily gay.
No I won't march in gay pride parade, I know I make out with dude but I'm only situationally gay man.