You’re absolutely correct. Some people “just know” and stay thay way, some “just know” and change, and some take a while to find out. There is absolutely no reason why that should be a problem.
My brother knew he wanted to be a doctor when he was five and he stuck with it. I’m 36 and I still wonder if I chose the right profession. Why is this considered normal for what you want to work with but not who you want to be with?
It’s because if you think you’re gay and kiss the same gender then discover you weren’t actually gay, you become tainted forever and upon death your soul turns into a cold sore in someone’s mouth instead of going to the Promised Land to play the harp with everyone else. Or something that makes about as much sense as that.