Why is it normal for men to hit women/girls on the booty?

The other day I was walking by my brothers friend and a girl, and the brother friend just slapped the girl on the booty and she didn't say anything. He did it again and she still didn't say anything, and I thought it was super weird<br><br>Later, I was in the cafeteria at school and I was eating with a bunch of people (friends, brother, random other people) and a male friend just patted me on the booty, said hello, and walked away. I slapped him on the chest and told him not to do that and the whole table started laughing<br><br>I'm just really confused as to why it's okay to hit/slap/push/touch a womans booty. I know a lot of men like the way women look and I get that, but women don't go around just slapping random men on the booty. What's up with that