Or the sex is so good that they hate each other even more for being so sexually compatible but so incompatible in every other way... so it becomes a sexual battle, lots of hair pulling, scratching, biting, and break-the-bed fucking. Orgasms all around, but in the afterglow they still hate each other's guts, so they promise two things: to never try to have a serious relationship together, and to never stop sleeping together.