I can't agree with this because I think a marriage is a unit for two.
When two join together, it signifies something much more than emotions bonding them together. It signifies trust. Loyalty. Honor for one another. Devotion. When you take time out of your day to be with someone, it shows that you care for them more than yourselves.
When you're with three or more people in a marital unit, things get complicated. You start choosing favorites. You can become distant from one spouse and chose the other over them. One of the biggest issues I see with multiple people marriages is that you don't have to work out issues with your spouse--you can just rat on them to your other spouse. In a two-person marriage, a huge part about cooperating and getting over hurdles is communication. If something bothers you about what your spouse is doing, you talk to them about it. You discuss how you can change it. You should expect your spouse to keep you accountable in this same way, too. But when another person (or more people) are introduced into a marriage, you don't have the same commitment to help each other; if something goes wrong with one spouse, just give them the silent treatment for a while and leave them in utter confusion and depression while you spend all your other time with your other spouse/s.
I have quite a few more things to say on this subject, Shawn, so if you'd like to stimulate the conversation with me, I'd love that!
Thanks for posing this interesting question.