It is possible for monogamous and polyamorous people to have successful long-term relationships. It takes major work on both sides. If a couple in a committed relationship cannot come to a true consensus about their agreements, there are major problems. Consensus decision-making takes more time, more energy, and better communication than compromises, but it is well worth the effort.
One thing that’s common in the situation, but that I personally think cannot have any place in any healthy relationship is a Don’t Ask, Don’t Tell (DADT) agreement. That’s just a way of enabling denial on the part of the monogamous partner.
Sometimes there is no real way to reconcile two people’s conflicting needs. If Frankie needs a monogamous commitment from their spouse but Morgan is not willing or able to be monogamous, they need to end their marriage as amicably as possible. If Frankie is able to fully accept the fact that Morgan is actively polyamorous, they will also be able to accept hearing about Morgan’s OSOs from time to time. In fact, some people find it natural that they would want to not only hear about those OSOs but to meet them because they are important to Morgan.
I’m a polyamorous woman in a relationship with a monogamous man. We are currently monogamous. If at any time I do not feel that I can continue to be monogamous, I will renegotiate things with him honestly, in a spirit of love. He knows that and trusts me to do so. I have been in monogamous and polyamorous relationships in the past and was actively polyamorous when we met (and very honest about it). I chose to stop dating other people—he didn’t demand anything. (If he had made such a demand, I would probably have reacted badly to it.)
If your relationship is important to you, take your time, and create sustainable agreements with your partner. Both of you must identify what you need, what you want, and what you can truly accept. There is no reason that you cannot both have your needs met, but you may need to be creative in finding ways to meet those needs.