Possible spoilers,
but I would love to hear your opinions on why Big married Natasha, and especially so soon after meeting her, when he had dated Carrie for much longer and also had said he was never getting married again. To me, it just feels like a plot device, but I'd love to hear other views.
I don't think it's shocking. He had intimacy issues, but was to the point in his life where he probably wanted a regular source of sex, life partner, maid, whatever guys marry wives for when they don't really love them. So picking a young and beautiful but sort of "basic" girl with whom you don't particularly connect, is a much safer bet than Carrie.
There are also hints throughout the series (though it's never definitively revealed whether it's the truth or yet another of Carrie's overly analytical conspiracy theories) that Big feels pressured into being with a fellow blue-blood, in which case Natasha also fits the bill.
It all seemed very '50s to me.