Swimming trunks

I'm now an almost 30 year old woman and this situation still causes me to feel so ashamed every time I think about it. I was 9 or 10 years old and a classmate and I were showering (in our swim wear) in the communal showers of a swimming pool during a school trip. I somehow decided it would be great fun to pull down his swimming trunks, and so I did. I started to laugh and make fun of him for how "small" his penis was and then told everyone in class. I feel so bad for disrespecting his body and - on top of that - for reinforcing harmful gender norms around the size of male genitalia