That girl is a paranundrum,ill never understand her!
The meaning of life is a paranundrum
What goes on in the mind of a woman is a paranundrum

"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.