Hypergamy is the word of the day.

Yes, men are also, to their own detriment, continually surrounded with images of exceptionally attractive women. But this has less practical import, because–to say it once more–women choose.

Or:

The decline of matrimony is often attributed to men now being able to “get what they want” from women without marrying them. But what if a woman is able to get everything she wants from a man without marriage? Might she not also be less inclined to “commit” under such circumstances?

This essay is not politically correct and at times it is misogynous and yes I believe the author is evil (seriously).  The main behavioral assumption is that women are fickle.  So they are monogamous at points of time but not over time; Devlin then solves for the resulting equilibrium, so to speak.  The birth rate falls, for one thing.  The piece also claims that the modern "abolition" of marriage strengthens the attractive at the expense of the unattractive.  Some of you will hate the piece.  I disagree with the central conclusion, and also the motivation, but it does seem to count as a new idea.  If you’re tempted, read it.

I thank Robin Hanson for the pointer.