Basically it's to do with power - rich old men buying trophy wives have been around for centuries. They've been subject to a mixture of contempt and envy - but on the whole they are a sizeable and pretty powerful class. I don't think the likes of Ruper Murdoch need protecting.

The other way round is a lot less common, and from what I've seen a lot more complicated. Where I'm come across it, it's seemed to me that so far as power is concerned the boot tends to be on the other foot.