Are we discounting that time I played doctor with my best friend and neighbor when I was three? I'll assume so
When I was 17; I had already been in two relationships with women and my best friend of two years (who happened to be male) asked if I had any interest in the opposite sex. He was non-confrontational about it and rather blase, which is a turn on for me, so I propositioned he be my first.
Bad idea, he accidently tore out my nipple ring.