The Economist article discusses the practice of widespread polygyny: that is, men who have multiple wives and whose wives are only married to them. As a matter of mathematics, polygyny (without polyandry) means that many men will stay unmarried, which makes them less connected to society and more likely to behave violently. That argument seems true to me.
However, the argument hardly seems applicable to the egalitarian polyamory almost always practiced by effective altruists. Poly female effective altruists can and do date multiple people. Further, many poly effective altruists are in same-gender relationships. If anything, polyamory as practiced by most effective altruists seems to reduce relationship inequality. People who have trouble finding a primary relationship can find a secondary relationship and receive many of the benefits of a romantic relationship. (Although not all, of course—I don’t mean to erase the very real loneliness that comes from having a hard time finding a primary partner, even if you have secondaries you love.)
Further, the article discusses polygyny in cultures where women are literally bought from their families by wealthy men. Both polygamy and monogamy are harmful relationship structures when women are sold by their fathers to strangers three times their age. That doesn’t mean that either is harmful when freely chosen by an individual in a society with much better protections for human rights in general and women’s rights specifically.
I think that relevant context for backlash against Davis Kingsley’s anti-polyamory views is that he is an orthodox Catholic. His anti-polyamory views are part of a set of fairly extreme views about sexuality, including being opposed to homosexuality, masturbation, contraception, premarital sex, and any sexual intercourse other than PIV. He has also expressed the viewpoint that polyamory should be socially stigmatized and people should be pressured into monogamy. I believe that much, perhaps most, of the backlash he has faced is due to the overall set of his beliefs and that it was disingenuous for him not to include this context.
Obviously, I am opposed to sexual harassment and to pressuring people towards any relationship style.
[Note: comment edited to use Davis’s preferred terminology for his style of Catholicism. The first sentence originally said “traditional”. I’m sorry for using terms for his beliefs that he doesn’t identify with.]