I label myself as straight, but I agree with that theory.
Sexuality being a scale, I believe very few people are 100% straight or gay. That means that identifying as bi is for many a matter of societal context and expectations shaping one’s perception. Say you on occasion find someone whose gender is not your preferred one attractive: you could either dismiss it as “I’m just noticing they’re attractive, nothing more”, or consider you might be somewhat bi.
What I find most amusing (and in a way very sad too) are those bigots that claim homosexuality or bisexuality to be a choice because, as they say, “everyone finds people of the same sex attractive, but you have to resist the temptation”. Like, dude, you’re super gay, just accept and enjoy it.