but first you need to tell Halsin you are somewhat interested
I fail at this point. The only line we can use to say that we are already in a relationship with someone makes it sound as if, despite this, we are kind of okay with it. During the walkthrough it felt as if the game was telling me that "any normal woman, no matter how much she loves her chosen one, would still be okay with "making out with a bear", and if she does, it's because she just doesn't like bears, but likes illithids, succubi or something like that, and if you don't need anyone but your lover and you resent such molestations, then you are obviously in the minority, and it's just a waste of time to write lines for people like you.
I decided once that I would do a "test", reboot, wash my hands afterward, and Astarion would forgive me, but I broke down at the "if you don't turn him down" point. That's what saved me from this preposterousness you described.