As a former Evangelical Polyamorist, now a born-again Monogamist, I enthusiastically endorse items 1 & 2 in this comment.

It can be thought of as the cultural equivalent of Algernon's Law - any small cultural change is a net evolutionary disadvantage. I might add "previously accessible to our ancestors", since the same principle doesn't apply to newly accessible changes, which weren't previously available for cultural optimism. This applies to organizing via websites. It does not apply to polyamory (except inasmuch as birth control, std prevention, and paternity testings may have affected the relevant tradeoffs, though limited to the degree that our reactions are hardwired and relevant).