It makes you look desperate.
People hate to say this, but sex can carry an element of power. It can be used as a weapon, in this case, of self-destruction. If she left you, and you go back to her for a few rounds of bump and grind, even if you tell her it means nothing (and it really does mean nothing), she will still have the smug satisfaction of knowing it was so good you came crawling back for it. Don't give her that, because we both know she doesn't deserve it.