The only answer I ever got in Sunday School went something like, “Well, those things happened a long time ago in the Old Testament, and Jesus changed everything.”
Only, where is the evidence that he changed anything? It is certainly not in the text.
Rather, it seems like the people of the Mediterranean who followed Greek and Roman marriage customs simply continued those customs as they created Christianity.
They took the customs for granted, as people do. They didn’t bother writing about it, because nobody thought about it.
Funny how that works.