The phrase is intended to encourage the woman to give up her doubts and have sex, because she wants it herself.
-Kevin, I am not sure about this...
-Come on, you know you want it!
cos ALL girls dont necessarily want it. It really pisses me off that you assume that all we want is sex or that "we say no but we really mean yes" it wont give up our doubts but it will give up our dreams (when you pressure us in the case of full-blown sex) so when we say no it MEANS no look it up in the goddamn dictionary.
Jill: I'm not ready yet, to give up my dreams of becoming a doctor we should wait a while, we're not mature for sex yet but when we are it'll be really special.
Joe: common you know you want it you prude
Jill: *slaps joe* grow up you pig and leaves joe with his right hand.