I agree. I'm of the opinion that monogamy is only harmful when it is considered the default. People/couples should put thought into this aspect of their relationship, and choose what is right for them. Even if that means monogamy.
The issue with non-monogamy is that a pairing (2 people) is the smallest working group, and that's what the game theory of dating proves to be most durable and successful. A 3-person working group requires at least 3x more relationship work, and a 4-person pairing requires 6x as much.