The idea is that people die before they can break up. While loving someone forever is entirely possible, it is undemonstrable, like the opposite. I think that the limited amount of time available to humans make them commit to an idea of eternity that just doesn’t belong to them. In relative terms, humans eternity is, aptly, their own lifespan. But since in the grand scheme of things, where nothing in existence is immutable, to assume that the desire to stay together and be attracted to each other is a possibility is, in my opinion, a myth based on an extremely small and faulty sample. Also, to know that we have a limited amount of time available creates the premises for an emotional survival economy. Meaning, just as an example, how many people feel like breaking up in their 60s?
Ultimately, I believe love is infinite and can last aeons. But I am also convinced that basically lots of people do their best to die before they can break up, as that is perceived as the only possible happy ending.
In this perspective, monogamy helps the survival economy, two people supporting each other in getting over the day, pass the night, and complete the given task: to not die alone.
If humans were immortal, they would break up all the time (they already do. I just mean more than now) and we’ll probably see that no one can stand someone else for more than maybe 100 or 200 years, especially if not plagued by the scary thought of aging, getting uglier, and being forced in loneliness (Incidentally, fear is another powerful motive for monogamy, if not the strongest).