So many interesting sub threads here.
I have a very traditional, "all-in" understanding of marriage, but that's because in my marriage, both of us are on our first marriage and brought little in the way of assets at the start. We have two kids together. It's not a blended family. Everything is neat, clean, and uncomplicated. Sharing our income and our net worth as communal comes very naturally.
When I look at my parents and the parents of my friends, though...upper middle age on seems like a hugely complicated snarl of crazy for remarriage. So much money baggage. So much emotional baggage. I don't think it's fair to make the same assumptions as those many have for couples just starting out in their twenties or thirties.
My mom was widowed in 2009 and she has vowed up and down that she will never remarry. Some of that is due to her temperament, but considering all the implications of remarriage in retirement, I don't blame her. Quite a headache.
I can see the attraction of a prenup in such circumstances. I'm very, very anti-prenup, but still, I can see why people would want one.