Why are people so quick to call girls sluts, but not boys?

I mean for instance, say a girl gets pregnant by her boy friend, or say a girl really is promiscuous, but so are the guys she is fooling around with, there are many different forms of what I'm trying to say, but in the end the girls still get the labels slut. The truth is, both the girl and the boy are wrong in the situation, so why do people only judge the girl? You hear about a girl sleeping with some guys and everyone is talking about her, but you hear about a guy sleeping with a bunch of girls and you wont even hear about. Some guys might even consider him a "ladies man". Or if a teenage girl gets pregnat by her boyfriend, she is instantly labeld a slut, but the guy is just the guy that got her pregnant. They both had , not just her. They are both wrong for sleeping around like that, so why is the girl always targeted?