I used to always ask my friends to show me their private parts
Why would you ask them that?
I just want to compare my private parts with theirs
It's not right to ask your friends to show you their private parts. They could get really upset at you.
They never complained so I guess they did not mind me asking.
Usually people do not want to see others privates and do not ask them to show them. It would be better to respect others' privacy.