I suppose the answer to this one is two fold:

1. Halsin is asking about a threesome specifically and not something related to Shadowheart - Can you imagine if someone you knew just randomly came up to you, while you were walking around town and as loud as you please said "So! I think you two a pretty hot - especially with you all wet and naked - how about a threesome." And this person has already tried getting into the pants of the person they're not addressing. That's some first rate stalker behaviour.,

2. Shadowheart has at this point said that they ONLY want Tav, and has given zero indication of anything otherwise - let's assume that the comments didn't also implicate Tav, that's the first step towards adultery.

There is just no way that this would ever be acceptable in a commited relationship - poly or no. When a person has said "no" that no counts, and if Shadowheart knows or not, is not of consequence regarding Halsin, who should honestly be slapped back to the grove and sit in a pen next to Kagha.

Also, when even the VA's found the dialogue troublesome, while recording it - the people who probably know the characters best alongside the writer - then something is messed up.