She treated you like crap.
You know those girl friends of yours who keep running back to their slimy ex because the sex is good, the ones who leave you shaking your head and repeating the same advice over and over again? You'd be surprised how often that person is actually a man. Women aren't always the innocent party when a relationship falls apart. Sometimes it's her unyielding iron fist that made it crumble.
If your ex lied, cheated, and was generally an unpleasant, cold human, we don't care if the sex makes you feel like you're Super Mario and you just swallowed a fist full of stars; don't do it. Not once, not ever. Leave her to shrivel up and die, horny and all alone.