So, now the story is over and it makes it easier to comment. 
I don’t think polyamory is the main dish here. I think the main point was social rules swap and portrayal of couples as something disgusting. It seems to me Murata really likes this theme, my impression is that she is really annoyed how people treats social rules are something sacred and she likes to play with what is seen as proper by her characters. She doesn’t give these characters freedom and happiness, she just changes what is regarded as good/proper and what as bad/weird to illustrate how absurd it is to cling to some stuff as The Only Right Way To Live.
sorta offtopic rambling about alternate forms of relationships being normalized or not normalized in the general society
It might be more Japanese thing, I can agree with that. It’s hard to me to say, because I was raised in environment where “traditional values” were very strongly valued so maybe it’s easier for me to empathize with Japanese characters rebelling about rules, because my whole childhood I was feeling that everything around me is very stuffy.
Poland isn’t a monolith in this regard, so experiences of other Poles might significantly vary. But the fact is that same-sex marriage seems like a fairly tale from far-away countries, something you only hear tales about I’m more used to wake up, read that some ruling politician said that LGBT+ people aren’t human (but ideology), shrug, and move on to the next thing planned for the morning. At it seems that the discussion here is more like “should we attack gays” vs “should we leave gays alone”, marriage seems so far away. I could hope for the new generation getting voting rights, but I thought my generation would be wiser and more open to differences between people when I was growing up. At it didn’t turn out to be. I was watching with horror as some okayish people became conservative and awful in their late 20s.
And somehow it is common to say that same-sex marriage would be favoritism, not equality. I never quite understood this train of thought. With all that, polyamory does seem avant-garde. But at least sex transition is possible here! It’s just awfully annoying to do.
Somehow this rant ended up being longer that the actual story commentary I’m sorry. But I’m so tired listening to how Japanese society seems so old-fashioned, when I get the same vibe when I look out of the window, so I wanted to talk about the different perspective 
And uh talking about rules I’m sorry if I’m doing something improper (
) but I really wanted to call up @Mithrillion on that topic because our media looove to compare Poland and Hungary in terms of LGBTQIA+ rights (or lack of them) and I crave some Hungarian insider perspective and I didn’t know where would be the right place to ask you this (if any) 