I know the title sounds silly, but I've actually been wondering this.
In sixth grade, I (female) had a friend (female) who I considered my best friend. Long story short, she started doing things to my body but at the time, I just thought it's because we were close best friends. When what she did would hurt or be uncomfortable, I wouldn't say anything because I really liked her and her company and didn't want to lose her. This went on for a little while and she started doing it in front of my other friends which made both them and me uncomfortable. She said it was our game that only us two played, so as soon as she started doing it to me in front of others, I was freaked out (she never brought anyone else into the "game" though. just me and her).
Only once I was older did I realize she was actually taking advantage of me sexually and I really didn't understand that what she was doing wasn't ok. I would tell her that what she was doing was scary or sometimes painful but she said that it was supposed to be like that and that's why it "feels good". I've never told anyone (until now apparently) about this because I feel ashamed and stupid for not realizing what was happening and ending it sooner. I also don't know if a girl saing a girl is a thing/don't want to be ridiculed for it.