cuck chair jokes aside, I think the "epiphany" he had in this chapter that most people seem to be missing, is that he's losing appreciation for the sex since all of them are so willing and ready for it all the time, zero resistance to it at all, whenever, wherever, they're ALWAYS down for, and extremely ecstatic about it, that Yuri offering the least bit of resistance, even if verbally, (and pretty weak resistance at that) made her the more appealing option in that moment. I think it's the same reason he was so happy when he fucked the siscon twin, she offered resistance, some friction before the sex.
Also reminds me of that one vid of a foreigner that's been living in Japan, and he was saying that one of the things there that shocked him, was when he went to bed with a girl, she started saying things to the tune of "no, stop it" he instantly stopped, and she was dumbfounded by him doing so. Then later someone explained to him later (I don't remember if it was the girl herself or a friend of his) that most of the time when Japanese girls say things like "no, stop it" during the deed, they don't actually mean it and want for you to stop and for them it's just a thing they do to "turn the guy on", and the foreigner was absolutely shocked when he was told that.
I don't know where to find it anymore but if I did I'd link it here.
Although I do agree having his mom and sisters there watching the whole thing was too much considering he's supposed to be someone from "our world" lmao