It's good when you and your partner know yourselves and each other. I always advocate for purposeful relationships of all kinds, especially monogamy. It has always seemed like monogamous people in general make a whole lot of assumptions; there's a unwritten "rulebook" that everyone is supposed to know by heart and follow, but each person understands it just a little bit differently, so arguments come about because of it related to things like what constitutes cheating (e.g. being attracted to anyone else, dreaming sexually about someone else, harmless flirting, etc.). If everyone engaged in conversations early on in a relationship about what they want and need out of a partner what they are willing and able to give back, and what their deal breakers are, then such incompatibilities can be addressed much earlier and compromises made.

And yes, my situation is very different. Monogamous-wired people who have threesomes usually haven't done the emotional work beforehand. They try to convince themselves that they are emotionally prepared for the aftermath because they are excited about getting some strange, but when the third person is more interested in one of them than the other in the moment, the other partner starts to feel jealousy and wants to shut it down. Or maybe one partner ones to do it a second time with the same third partner and the other doesn't because they worry that their partner is developing feelings. Or any number of things.

With the people in my circles, we all have practiced polyamory for many years. Sometimes we experience jealousy (I don't), we have done the emotional work to identify it, put a name to it, dig deep to determine the cause of the discomfort, take ownership of the emotion, let our partner know about the feeling, and ask them for what we need as a means of support without imposing on their boundaries or attempting to force them into changing how they engage in their relationships with others.