I actually think women are great at sensing when a guy is "hard up". I don't mean no job, live in your mother's basement, eat pringles and pizza while you watch TV "hard up" (to be blunt, you don't deserve any tail if you do that). It's just that when an otherwise normal male who regularly engages in sex goes without (i.e. girlfriend away) we seem to change in a way women can pick-up on. It's a phermones thing I believe.