Is it gay to accidentally fap to a transgender female model on instagram?

She did hormones and had plastic surgery, I assume, because she simply LOOKS like a girl. But on her childhood photos you can see she used to look like a boy.

You're misinformed. If hormones are taken before puberty the physical male features do not develop. In Thailand the lady boys are very convincing. It's very difficult to tell the difference if not impossible. You have the impression that they all look like men in wigs but that's because those ones standout and don't pass. The ones who pass you don't even notice or you probably thought they were hot.