IMHO, poly is fine when everyone is aware of it, thus, if it's even possible, everyone should be getting along with everyone and that's what makes it a pipe dream 99.9% of the time I guess, Rentarou representing the 0.1% with his godly blursed situation! Contrary to him though, I despise that new character, I don't sense a iota of respect for others out of him, he only thinks of himself. Like someone here already pointed out, he seems like a narcissist and sociopath. Dangerous cocktail
Also also, why does everything have to revolve around sex? Isn't there a huge part of relationships that's about something else. What if heaven forbids there are people fine with none or very little of it?
And finally, I would like to virtually slap in the face everyone who calls Sayu names that I won't dare repeat. God some people are assholes, not just in mangas.