With each romantic relationship, you get a different experience, and different things get emphasized. It's easier to tailor the needs to the specific person if you are one on one, and negotiate ways to get around unmet needs. But in a poly, you are going to have to shift the needs constantly between partners, and someone is going to get left out. For example, you may be better at fulfilling Partner 1 sexual needs, and Partner 2 emotional needs, but 1's emotional needs and 2's sexual needs are going to get left out unless they can cover for it. If you have more than 5 people in your"polycule" you are going to get clusters of people favoring each other since they can better cover for the unmet needs.
Even if there is no cosmic law that says you can only love one person at once, you do have physical limitations to the needs you can give and what you can manage that makes one partner only more practical for most people in most cases.
I find that a lot of these SJW types don't really have much experience in actual relationships. They base it off concepts in Tumblr, where everything is always neat and clean.