The Freakonomics blog has a post titled "Why Do Beautiful Women Sometimes Marry Unattractive Men?"
Gee, why do they?
Apparently it's not always down to economics -- at least, not in the way you're thinking:
A new study by Satoshi Kanazawa, an evolutionary psychologist at the London School of Economics, suggests it may be a simple supply-and-demand issue: there are more beautiful women in the world than there are handsome men. Why? Kanazawa argues it's because good-looking parents are 36% more likely to have a baby daughter as their first child than a baby son—which suggests, evolutionarily speaking, that beauty is a trait more valuable for women than for men.
The data for this study must be all kinds of slippery (How do you determine that parents are "good-looking?" Except in obvious cases, do attractive parents always have attractive kids?), but I would like to point out that mental_floss' devastatingly handsome president and his super-hottie of a wife just had a baby girl, which is unshakeable evidence in the second point's favor.