Nuclear families are (or were until no-fault divorce) traditional. As you go into the past, with different conditions, extended and substitute families happened, which largely provided the same structure as a nuclear family.

Extended families are still traditional almost everwhere but the US. We are the odd family out, and I would argue that the extended family offers more security and a better environment than the nuclear family. Look at Charlie’s story as an example. Exactly how the modern nuclear family came about would make an interesting study, and there is probably a tome out there somewhere, but I doubt the reasons are because of traditional values.

Actually, can you name any polygamous societies that treat women in a fashion that could be considered acceptable in Western society?

No. But as a counter example, I offer up Europe of a century or two ago. Much has to do with the spread of modern civilization and it’s liberal values. Whether or not civilization and liberal values are tightly bound is an open question. Certainly, classical civilizations required no such connection.

As to mistresses, I would argue that they are a form of polygamy adapted to a monogamous society, to the detriment of the bastard children.