I think part of it is that the game doesn’t recognize the Shadowheart romance as fully settled and official until act 3? So you still have all of these people propositioning you in Act 2 like Halsin, or Wyll with his dancing scene. Now as someone who also romances Shadowheart, being hit on by these guys is definitely unwelcome, but maybe other folks like that and want to see those romance scenes as well.
Some characters will acknowledge that you already have a romance if you choose to spend the night with someone else during the tiefling party. For example, Karlach accepts that you have a different romance interest (and she talks specifically about Shadowheart) and is very respectful from then on, not making any other suggestive comments. Lae’zel complains about you picking someone else but then she moves on so it seems. It’s the guys who keep hitting on you, and like I said, it may not be welcome for some of us, but other folks do want to see that. It would be cool to have an option to toggle romance specific dialogue on and off for certain characters, but that would be a lot of work for Larian.