I honestly think it comes down to control. Women these days don't need a "Man" like they used to. They have the courts and police force to do their bidding and 50% of the workforce is now female, so they can make their own money and pay some guy to fix whatever they need done around the house.
So, it comes down to control. Most women, not all, who marry a younger man do so for issues revolving around their ability to dominate the relationship and stay on top. I personally know 4 married women who are older than their husband and they are all control freaks. Some have even said that they feel more like his mother than his bride. I don't know, but perhaps some women just want a "boy toy".
I know this "cougar" thing is gaining some traction, but back when men were men and women were women, a gal wanted a man who was a bit older and wiser, but it doesn't look like those things are as important to females as they once were.