Homosexuality: How did you know?

Sounds like a pretty stupid question, but how could you be 100% sure that you are, indeed, homosexual?

I'm currently debating this with myself. I've gone through straight-phases and gay-phases for many years. Sure, I could probably just slap the bisexual label on and be done but I'm now disgusted at the thought of heterosexual sex. The thought of lesbian sex excites me and gives me butterflies, without me even picturing a specific person. So... what gave it away, for you?