Originally Posted by Cawyden
As long as you don't have the orgy with him and the drow twins you are not forced into it. But with the drow twins he transforms and I don't think you can prevent that (not sure if it is different if you have him with Shadowheart or Astarion).
In case of solo romance, Halsin transforms anyway, but you can ask him to turn back to elf. In case of having SH/Astarion he does not transform, they just have comments about each other.