A situation of twisted circumstances where you are unhappy if you do the right thing and you are equally unhappy if you do the wrong thing.
"Dave" loved his girlfriend, but he also loved the things this other girl would do for/to him. If he stayed faithful, he'd miss his opportunity for a wild hook up. But since the other girl was his girlfriend's sister, if he took advantage of the hook up he'd loose his girlfriend. Dave is in a sexual paranundrum.