I think that we're defining norms in slightly different ways. I have been meaning norms to mean "what people should do" and not just "what people do." I've also been stretching monogamy a little to mean the ideal of having only one partner at a time, whether or not that's in the context of marriage. It's a fairly common usage, but still easy to be unclear about.

So I would say that serial monogamy is still monogamy, and that promiscuous sex outside the context of any relationship has little to do with relationship styles at all. Your examples point to a failure to enforce monogamous norms while the norms remain unchanged, I would say.

Or perhaps monogamous norms are weakening, and these examples are the result of particular groups not having any strong relationship norms at all; various kinds of polyamory and swinging offer social norms that can be as strong as monogamous ones, but they're largely unknown.

Again, it's hard to say. What evidence there is can be plausibly interpreted in quite a few ways, some of which I'm sure neither of us have thought of.