Where did you get that from? I'm not saying that, not at all.
Do I think that living and open relationship (not necessary polygamy, because the terms implies numerous relationships each with the same affective and contractual status) is the best way to go? Yes.
Does that imply that humanity, freed from the religious oppression, would go for that? Not at all. There might even be some biological bases for monogamy, but we are rational beings. We can work around our instinct. We do that all the time. Some of us can successfully do that for monogamy, too. It would be great if that could be more largely accepted.