But, they put SH in this situation to sleep with Halsin and their Tav's aren't mono either after all the bed stories. If they break up because of a flirt or a dream, then there is something wrong with the relationship.
The real relationships thing and whether you think the break up is warranted or not is entirely up to you. But I rather not go deeper into it than necessary, it has a tendency to derail the topic.
I think you are being a bit selective in using peoples arguments with what the issue is here. The flirting/invitation to sex in that banter happens regardless of whether or not the MC opened the relationship or not.
I'm just going to recommend you re-read the posts (at lest the recent ones) people have made in both Shadowheart and Halsin threads. (Ninja as someone that has a different perspective than most had some good posts recently that they made in regards to the writing/implementation) And if you still disagree? Then we just have different views on relationships and in this case the story/writing.