It's a tough but really interesting subject that touches the relationship between the nuclear family and private property. I'll try not to write a whole book so I'll focus on what you need to explore to understand the whole infidelity paradigm.
Firstly, observe how much housing and property is tied to family status. Ultimately, marriage is not a love affair it's an economic one.
Secondly, explore how the reunion of private property, love and sexuality is a historical construct that has and had many alternatives.
Finally, understand how social constructs corners us into absurd dilemmas. Cheating is dishonest, yes we should always say that we want an open relationship before lying to somebody who trusts us, unfortunately, that's not how things present themselves in reality. Your uncle might be an ass, idk, or maybe he knows wanting an open relationship would mean divorce, aka, personal level economic crisis, and that nobody wants this, so cheating is the lesser evil. You might say well just give up on sex/love if you're married, but I don't think we should expect people to renounce that very prevalent aspect of the human life because of some private property shenanigans
Hope that helps seeing a different perspective, I personally got way more involved in questionning love/marriage and family after I experienced living together as a couple. Some stuff you gotta live it in your flesh to understand why people do the things they do