Maybe I'm becoming too open-minded or I don't know but the sex between Katra and Io felt super natural and right to me. That's the point, nobody is nobody's girl, everyone is truly free and loves every member of that village equally. This little act of sex was just an act of friendship and Mira expressed it really well. The two had ideal chemistry, they were thankful to each other and happy that they talked, so sex was like...buying a delicious snack for your partner - a wish to show one's gratitude, to deepen the bound between them. And it was honest because both Katra and Io treat sex as nothing more than showing all sorts of love: for a friend, for a lover etc. Now if Juna hooks up with some random chick - that would feel unnatural.
It’s not that I don’t get the polygamy aspect of the manga. I don’t mind everyone loving on each other, but my interpretation of Io is that she wasn’t just sleeping with anyone but Kidaha. I feel like she’s completely devoted to her. It’s like watching all the other established couples randomly have sex with a different character. If everyone’s just sleeping around anyway, then what’s the point of even calling themselves “Kidaha’s wives” or bothering to separate from Kidaha when they get together with one another.
Also I don’t recall anyone just messing around as just friends in the previous chapters. (Aside from Katra) Either they expressed love or fell in love after. I’m not saying characters can’t fool around if they want, but aside from kidaha it doesn’t seem like they’re all doing that. Also, Also, going from “I love kidaha, please leave her” to “ok let’s have friendship sex now” caught me off guard.
