Halsin doesn't have quite the same problem, I think, since he's not written to be poly or mono or anything in particular during Acts I and II. We learn that he's sometimes open to flings, doesn't currently have a lover and is interested in the protagonist - seemingly only the protagonist - but not willing to act on that interest yet. He does still show interest even if you lock in with one of the others, so I guess that could imply... something. It's only Act III where things really get messy.