Perhaps one reason why it takes some people a long time to realize that they’re not straight is because conventional beauty standards about what is “hot” and “not hot” just don’t always resonate with them.
You could show me an image of the Sexiest Woman in the World as voted by all straight cis dudes in the world and I might think she’s pretty. I guess. Maybe if I got to know her as a person, I would see the appeal… but she’s not really doing it for me right now.
If you’re not attracted to this walking talking Aphrodite, this ideal of a woman, you must not be attracted to girls, right?
Anyway, maybe eventually you realize that your tastes just don’t match up to conventions – at least not heterosexual conventions. Maybe traits that general, straight society maligns as “ugly” are actually really appealing to you.
Or maybe it’s as simple as realizing you’ve just… never actually been the intended target of a narrative of attraction. Commercialized sexuality hasn’t been created with you in mind – or if it has, it’s meant to cater to straight people at the same time.