I myself am poly but I've been in a relationships where the other person wasn't comfortable with it and agreed to monogamy and have been on the giving and receiving end of infidelity. As I've gotten older I've understood the damage I've done and how to properly engage with those feelings and have an open and honest discussion. I think we actually agree more than it seemed. Feelings aren't choices. Actions are. And the social punishment for failing at monogamy is often brutal and unfair. I just think two people can still freely say 'don't act on it and if they both mean it, breaking that hurts. That's all.