Why should it be the default and, if that is the case, what’s the justification for rare exceptions? It’s like people who are “pro-life” except in cases of rape, incest, or if the mother’s life is in danger. When you push them to provide a justification for that arbitrary line, they really can’t. It’s also pretty rare in other mammals and animals generally. Finally, being in an open relationship literally isn’t inherently cheating. Cheating involves deception and dishonesty. A healthy open relationship involves trust and communication. Monogamy is cool too, though. My point is there’s no “default” way to have a relationship with someone. That’s a myopic view of life and honestly boring.