Why do men stick together but women put their gender down?

Like if a guy sleeps around, his friends will be happy for him but if a girl doesn't it, her female friends hate her and call her a bunch of names. If a young girl is pregnant, females call her names, judge her, etc. A guy can show skin or be nude, he doesn't get name-called by other people but people esp. FEMALES judge harshly a female that shows skin or poses for Playboy.. Why do women/girls do these things? What happened to being there for each other??? I mean, she's didn't kill your parents or stole your husband in order for you to bash her..