You guys seems to forget that the main issue is how poorly Halsin inclusion done . Her acceptance to open relationship can be somehow justified. However, the way Halsin is forced into conversation and SH goes full Daddy Halsin mode in 2 dialogs is OOC.
Given how she flirts with the other companions in the early acts, it wouldn't be entirely OOC for her to do so with Halsin. IF it wasn't after you've established a committed relationship with her and it didn't involve him offering to join in on your intimate moments.
She could have lightly flirted with him, like the others, before act 3 but she was too busy hating him in act 2. I wouldn't have cared much if she had flirted before tbh, as its clearly just for fun. But the timing in act 3, the way it involves a highly intimate moment between the two of you, just makes it awkward. They definitely need to get rid if that banter, as it just doesn't fit with the times.
It's even more awkward when you remember that he tries to get both("You and your love...") SH and player, especially if you rejected him before. A bit creepy, isn't it? Her response doesnt help either.