How to go from being straight to being gay
You probably assumed you were straight because that was the normal thing to expect. My experience growing up was somewhat similar. Up until around eighth grade I assumed I was straight, even though since I'm a late bloomer I didn't really have any sexual desire. But I still would have little crushes on girls. It wasn't until a years after becoming sexually mature that I realized I was attracted to guys. So I went from straight to bisexual. But oh wait, after a while, my attraction to guys became overwhelming and I was no longer interested in girls at all. I went from bisexual to gay.
I can't say exactly why this happened, but I think it's just that we had assumptions and expectations of ourselves that just made us think we were something else before we even got time to find ourselves. For example, little comments and questions like "do you have a boy you like?" over the years and simply being around people and thinking you're the same as them. Maybe this is some type of unintended indoctrination.