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

Chill, dude. You're way too worried about an invisible nametag that you think you have to wear, when it doesn't even exist. I suspect you're also preoccupied with appearances and how people present on the internet.

Sexuality is complicated, but most people these days should understand that sexuality is a spectrum, and not an either-or thing. I realised a very long time ago that I'm way over on the het end of the scale, but I can imagine being sexually attracted to some hypothetical guy. I've never actually met a guy like that, but I accept that the possibility is there, and I don't see how that makes me in any way gay. I wouldn't be attracted to a man, I'd simply be attracted to a particular person who happened to be a man.

Despite what lots of people want to believe because it makes things easy, life is not all black and white. Your interests are what they are, and as long as you're not acting in a self-destructive, dishonest, or abusive way, you should just try to accept that you are who you are, and the people you find attractive are just the people you find attractive. You shouldn't let anyone shove you in a neat little pigeon hole, and you shouldn't cram yourself in one either.