In a weird way, I can see how everyone can kind of tie into a poly relationship. Even Monday felt like she has a good relationship going on not only with Haruomi, but with Tuesday and Wednesday as well.
Diana1504 and thefreeair's comments seems pretty biased because they're assuming that Haruomi is tied by a mono relationship standards, BUT the problem is, he is not. He clearly set out his expectations from the beginning and honestly, it was up to Monday whether or not to break up with him at that moment because she had these expectations of the monogamous relationship. BUT AGAIN, the problem is she didn't.
If anyone's a bit off putting its probably the main character. I can truly understand why she also feels like monopolizing Haruomi. In fact, she reassures other weekdays that they're well-loved by Haruomi. It's just really off putting how the mangaka kind of does little to hide the thing where the main character is special because of plot.
It just really jarring because with poly relationships, its a huge no-no to fuck up the entire relationship just so you can be exclusive with one of your partners. That's just usually a huge fucking mess waiting to happen. I guess the only problem I have, is that---- no you shouldn't have expectations to be exclusive with someone who clearly isn't interested in being exclusive.
But thankfully, plot saves the day by explain that Haruomi is looking for someone to be exclusive with and Monday is the girl lucky enough to roll into the spot. In the end you should just treat this literally as a dude dating around in high school because its a time to experiment and see what you want.
I honestly don't see anything that Haruomi has done yet to deliberately emotionally or physically fuck with people in my mindset, unless of course you're bogging him down with traditional relationship standards.