I was sexually attracted to girls and not boys.
Okay, so you established your attraction more to women over men. That's not the same as establishing a status, as you hadn't yet made a choice to be with women. Until you make the choice, no one knows for sure what you may do.
Logically, I knew I was straight.
You "felt" you were straight based only upon your attraction.
Logically, you hadn't acted on that attraction, so you hadn't done anything.
Again, it would be like being arrested and convicted for a crime because you admitted to police you had thought about committing a crime but you never actually did anything.
I didn't have to actually have sex (or kiss or whatever) to define my sexuality.
You're confusing what you "felt" with what actually happened and wanting to make them the same thing.
You "felt" you were straight and thought that defined your sexuality, but logically, it didn't. You just "felt" it did, and that's not the same thing.
How old we're you when you made that decision?
I don't actually remember how old I was when I had my first romantic interaction with a woman. The point is that until that happened, while I felt more attracted to women than to men, I could have made a different decision - the same as anyone else can.
Just because you are more interested in one gender doesn't absolutely prohibit you from choosing the other one, as much as you might feel otherwise.