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.
And hey, Io remains Kidaha's most important woman, so why would she leave? Who said that Kidaha loves Katra the same way as Io? It's obvious the reason is Juna, if Katra paired up with Mifuri and continued to live happily with everyone, Kidaha would treat it the same she always does. But here came Juna - equally strong, arrogant child who rivals Kidaha in her leadership and wants to take one of the girls away, forever. Anyone would get their pride hurt and make a mess out of it, especially Kidaha. So I don't see any problems here.