I'm simply not interesting in why men do it. Probably because I assume the reason is usually sinister (sex, control issues, money). Before I make similar assumptions about women I'd like a little LSD feedback. Oh, and one of my female friends just married a much less intelligent guy; here's an actually conversation we had a few days back:
Him: Hey, guess where fireworks were invented
Me: Asia
Him: No, China.
His wife: Honey, China is in Asia.
Him: Nuh uh.
She then proceeds to explain what a continent is. He's almost 30.
Maybe the sex is great. Maybe he has a nice, juicy trust fund. Maybe he lets her walk him around on a leash and whip him.

Srsly though, I think a lot of women are forced to "settle" for the next best thing. As annab alluded to, its really hard to find a guy who
really doesn't mind being with an intelligent, ambitious women. The fact that you're going to be working 80 hours a week and not have time to cook, clean, wash, iron and ego stroke doesn't sit well with a lot of men.