Why is it that in society it is normal for guys to have sex and such but if girls do it its wrong?

i dont understand... being a thirteen year old girl i try to keep all my sexual thoughts to myself but i was thinking the other day how guys seem to get away with more things like masturbating, sex, etc. when girls are taught that those things are considered shameful and ur a slut for doing it but guys are cool if they get into that?? i just dont uderstand... i want to kno y this is.