Two words: relationship escalator! We are programmed (in a monogamy based world) to expect that everything has to go forth in a certain manner. If you are both sexually compatible and emotionally attracted to someone, you have to then become exclusive, move in together, get engaged, get married, and pray to heaven that the feelings you have now will last for the next 50 years. Those are the only real choices that you have or so you believe. “If we love each other, we should cohabitate for the rest of our lives.”
Not that everyone should become poly, but there are some things that might be taken from that world about just letting relationships find their own level, rather than demanding that they continually escalate towards the goal of legally sanctioned cohabitation for the rest of your lives. Of course, that’s easier to do when you are permitted to have both sex and love with more than one person.