Why are some guys like this?

There's this culture with some guys, they're like 'sh*gging birds', and the whole objective is to sh*g as many birds as possible, my housemate is like this, he's cheated on his girlfriend and is very promiscuous now he's single, yet he actually seems okay as a person, like to talk to and that. BUT here's the thing, he's 21, and has a sister who is 16, so what now? Does he want guys to sh*g his sister for another number on their number of girls list, just as he wants to do to other girls, and wants his mates to do to girls. He also said "I can't wait to see my dad's reaction when my sister brings home a boyfriend", as if the poor girl isn't aloud to have a boyfriend and bring him home, when he himself brought his girlfriend home several times. Put simply, he's a moron, he sees it fine to cheat on his girlfriend (while he'd be outraged if somebody did it to his sister), and he has sex with girls promiscuously, but sees a huge wrong with his sister doing it (if she does in the future). Also, he implied his dad would react funny to his little sister's boyfriend, when him having a girlfriend was completely fine. Double standards left, right and centre.