Originally Posted by Ixal
The difference is that Astarion was always planned as a companion/origin.
Halsin was only made into one after a lot of demand. So his story was never set up for deeper interaction past act 2 and instead of rewriting him and larger parts of the story Larian just focused on the aspect demanded most. The thirsting.

In this case they could have just written him as fling - casual sex for Tav no feelings required/included. But they didn't. Even if you only want him for casual sex he will still tell you about his stirring heart and you will still get romantic dialog with him. They wanted him to be both - romance and fling but right now those who want a fling get romantic dialog from him and those who want a romance get dialog with him that fits better to a fling.