Ooh, ooh, I've got one.
My wife's nephew, who quite frankly is a simpleton, started dating a woman (both in their early twenties) who was one of those "no sex before marriage" types. I never could figure out the attraction because they were so completely different from each other. Yeah, yeah, opposites attracting and all that, but this was just too opposite. While she was just slightly overweight, she was still quite attractive, had an outgoing personality, and was fairly level-headed and responsible. She even owned her own home. Him, no so much. He's rather skinny, shy, unassuming, not all that good looking and, as referenced earlier, dumb as a box of rocks.
Anyway, they end up getting married and she gets pregnant almost immediately. About a year later, just after the baby is born, she turns into the bitch from hell, kicks his ass out, and makes a deal with him that she won't seek child support if he'll sign away all parental rights to the kid and promises never to try seeing him or her ever again.
All it seems she was after was a sperm donor but because of either personal beliefs or parental pressure had to be married in order to get pregnant. I figure she saw that she could manipulate him and get what she wanted without too much "inconvenience."