Very well. Here's one of the three I mentioned. You find the others on youtube.
SH makes it clear that she would have been okay if Tav asked first. Asking first = setting boundaries on the relationship = ethical non monogamy.
Edit: would it have been to have a longer discussion about this in one of the "I want to talk about your relationship" options? Yes. Do I want Larian to revise that in lieu of fixing the end game? Hell no.
And there we go, scene number 1.
You say that she has no issue with sharing, she even goes as far to say that she isn't looking for 'something loyal and pure, a swan' but isn't happy with you going behind her back (cheating - which surprisingly in this case she is willing to overlook without breaking up unlike with the others.) And yet she rejects sharing with the same line 'the swan' from the player if you were to initiate sharing/poly dialogue concerning other companions/npcs.
'The way I was raised, the way I was trained...well. It was positively encouraged to get to know each other.' Is her other line in this video. Does casual sex indicate polyamory? If it did everyone would be classified as such so, no. That scene tells me that she is open minded when it comes to sexual experimentation at least. (Which again, changes for this scene but not for others.)
My issue with this scene and my statements in the past that it contradicts what she said before comes from her ending things when you sleep with others. ( I might add the breakup still happens if you decided to pursue someone else that isn't Halsin after the Mizora scene even if you tell her first.) Its not that she might be the way you claim her to be but the way it was handled/written.
The scene and the writing in it are a clear (to me) example of a late addition to the game that goes against what was established throughout multiple conversations you have with her throughout act1/2 and partly 3.
I mentioned this before but if a scene popped up out of nowhere in act 3 where Wyll was saying that he was cool with killing innocent refuges/civilians it would stick out the same way Mizora scene does.
Didn't see that edit. so here's mine - Yes the option 'I want to talk about our relationship' and establishing what the player wants (and if that's possible with this character) would've been a great addition but then again this game lacks so many things, especially in the writing department. (I don't think there is a single character in this game that was well written, the focus was obviously on making them waifus/husbandos.)
