Well, if you look broader, all of the companions have a figure in their life that they are fixated on, be it Mystra, Mizora, Cazador, Shar or Vlaakith. I think this is because of the origin stories being custom tailored to the main plot of the game. My guess is that the absolute will use that figure to try to bring the companions to its side.
So whether Gale talks about his goddess ex-lover while Shadowheart about her devotion to Shar, or Astarion about his hatred towards Cazador, they all might face the same turning point when the party confronts the absolute.
Shadow sees a man in her dream, not Shar. Yes, they all have someone, but the level of relationships is clearly different. Astarion also has a "nightmare", not a "love dream".
Shadow in her romance does not talk about this man, Gale discusses about "goddess" constantly. You can even ask him, " do you still love her?" I don't like such romance, it reminds me story with Red Prince, do you know how it ended? Awful.
So I wouldn't even called these romance options, just sex options.
Shadow and Gale dont have sex scenes, their cutscenes look like the beginning of a romance. Astarion and Lae cutscenes look like just sex for fun, I agree with you here. That's probably why they're the easiest to get.
