I'm not gay, so forgive the intrusion, but I think my answer here will be valid.
I never "realized" I was straight. I don't think any straight person "realizes" they are straight any more than gay people "realize" they are gay.
I think we are attracted to who we are attracted to, and that just is what it is. Societal "norms" and pressure certainly play a part, as in it is considered "normal" to be straight, so gay people are often oppressed and made to repress their natural attractions in order to "pass" or "fit in" or whatever. But, at least in some parts of the world, historically there have been times in society where gay/straight were more fluid... people were more bi-sexual and more accepted in those times and places.
Religion plays a HUGE part in the repression. I mean, reproduction is the primary reason to be straight, since outside of modern medicine that's the only way to product children and have more people in the future! But just for dating, having fun, etc, there's no reason people can't be gay or bi and it be perfectly fine and no one should have any issue over it... except... when religion drives society to establish "rules" then sometimes that means gay is "bad" because of whatever religion reason.
But circling back... I never "realized" I was straight. I just at some point had a first crush on a girl, then another, then another... and I've always and only been attracted to women. Doesn't mean I can't appreciate the types of men generally considered to be attractive to others and acknowledge it... but only women move the needle for me. It's just how I was born, how I'm wired, and I've never had a reason to question it.
More importantly, I happen to have been born the more common way in modern society, so nobody questioned me... except, they kind of did.
I actually had the gay "experience" growing up without the benefit of being gay. Let me explain.
I've never been a player, ladies man, etc. I was never in a group of guys and talked about girls/women, I never hanged pin-up posters on my wall or talked of "what I'd do" to a girl... just not wired that way... so, when I was in 5th grade and someone started a rumor that I was gay while I was out of school for two weeks with strep throat/scarlet fever... I came back to a surprise school of kids that suddenly hated me, didn't want to accidentally touch me, etc. Boys and girls alike shunned me for years until I moved and started at a new school. My early social development was really fucked up as I got all the same treatment (shunning, name-calling, isolation, random beatings, etc.) that an actual gay kid would have gotten... only, I wasn't gay. So I didn't even have the possibility of the gay community being a rock for me. I wasn't gay, had no way to "prove" I wasn't gay, and learned how horrible all the kids around me were and frankly wouldn't ever have wanted to belong once I saw their true colors.
Long long story short. While being straight, I've always felt a kinship with the gay community because of how I was treated growing up. And circling back to the topic from the OP... I know some people have confusion about their sexuality, but I think that is less about their own "realization" and more about how much pressure they have on them from society to doubt their natural feelings and repress to fit in... I don't otherwise think we have to "realize" our sexuality. We are just born how we are born and as we mature and have those first attractions, that's when it becomes obvious.
