My personal experience is that sexuality is more fluid than people like to admit. Well, either that, or a lot more people are bisexual than they like to admit and it just comes out when you meet someone you feel so strongly for you can't ignore it.
I've got a male friend who defined as straight until his mid twenties, then decided he was gay but had always been in denial, and then in his forties somewhat sheepishly admitted he was bisexual as he'd met a woman he wanted to be with (first in 20 years).
I have a female friend who ID'd as straight until she got divorced in her fifties, then met a woman and fell madly in love and now ID's as gay, but seemed very happy with her exH for many years.
I have at least one female friend who has changed her self definition four times at last count and now mostly just says she's someone who falls very very hard for whoever she is with and can't imagine wanting anyone different.
Randomly, have you seen stats for the number of people under 25? Less than 50%, in a recent study, identified purely as straight which I think is awesome. Even if most of those end up largely in heterosexual relationships, I think the world would be a better place if straight wasn't the norm, if we accepted that someone might be 90% straight and 10% not and it wasn't a big deal if one day they met that person that made them want to act on that 10%. It makes me really hopeful for the future.