Why am I gay?

I'm 19 and I didn't really know I was "gay" until recently. Honestly put it off as being bisexuality for two years but I realized that wasn't right.Problem is I don't really know why I'm gay. I don't think I was gay before like, 14-15, never really felt any same sex feelings. I've heard that sexuality is determined by actions in your childhood? I've been reflecting on my life and kinda remember when I was really young taking a bath with another boy and kinda doing sexual stuff. Would that be enough to tip the scales? I also think I called a male 'beautiful' when I was 4.I've been super skinny ever since puberty, being underweight this whole time. Maybe lack of testosterone or something? Feel really uncomfortable about being gay. I don't think I really appear manly enough to my family or friends. Being 'out' would just make me feel weak or vulnerable.I thought about having to introduce a boyfriend to my mom and dad and all I could think about is what they would be thinking, like "Oh our son gets fucked by that guy" which really freaked me out. I also have had frequent arguments with my mother all the time since I was like, 9.What do I do?