I never was! I actually liked girls first (or at least that's what I still think it was), in school but like in secret I never told them.
It was in high school when I found out what sex between men was, and I really liked it, but I acted like it was just not that important. And I kept telling to myself that I was straight, that I will marry a women, have children and all of that.
Last year I accepted that I was gay, but I never even had the thought of being bi because I basically don't even care about women anymore.
Even when my mom tried to out me! she asked If i was bi, I said I like girls but in my mind I knew it is not true, I like boys.