My interpretation of monogamy is not static at a point in time; it implies faith and trust in the person you're having the relationship with, and to a certain extent forgiveness for flaws that will be apparent once you have lived together for long enough, and they usually (not always) break down precisely because one of the two wants a different sexual relationship with impunity. There's a reason vows make you say "till death to us part". Divorce has sort of developed to accommodate this, accepting that this promise will quite often turn out to be a fiction because there are valid reasons to break off a monogamous relationship, but the ideal has remained (for reasons much too complex for a forum post), and that process of realizing that, no, actually, this cannot go on is extremely painful to go through.
In the other part of my post I was hinting at the way some people use an "open relationship" to indicate they want out, but don't have the courage to break it off completely. That way is extremely bad and probably explains a lot of why open relationships have such a bad name. But many people in open relationships have realized that they are not built for the monogamous trust model, and so are trying to build a life together with multiple people without having to cut off their earlier relationship, preferably with as clear and transparent communication as possible (which is ridiculously hard work tbf).
