"a guy is the key, the girl is the lock"?????

"imagine this, a guy is a key; and a girl is a lock. it is good for the key to be able to open many locks. but for a lock to be opened by many keys, it is not good news." Su Ann quoting someone.....

i dont understand why it is perceived this way. but that really seems to be the case. even in America where people seem to think is the best place for open-mindedness as they practices Freedom of Speech, singer Christina Aguilera sings in Can't Hold Us Down

"The guy gets all the glory the more he can score, While the girl can do the same yet u call her a whore". the lyrics clearly states how people everywhere practices double standard.

why do people do that?

*pouts...i don't get it..

can someone please explain this to me?