Have specific preferences could be interesting beyond characterising the npcs though; Astarion for example was (I'm assuming) forced to seduce anyone his master took a fancy to. There could be interesting things with that; instead of "let's make the gay character straight" or vice versa, emotional conflict could be mined from it; how much of his attraction to the player character is simply due to Cazador's forcing him, conditioning him to go after that gender as well, and how much is genuine attraction? I think that would cost too much in devtime to do anything with even if I think it's got an interesting 'hook' to it.
Absolutely, and someone with that sort of history could go in a lot of directions, many of them quite interesting. He could engage in compulsive sexuality while being emotionally dead to anything beyond that or conversely he could be entirely disinterested in the subject of sex at all, or even traumatized to the point that he could not bring himself to share any aspect of himself with anyone ever again.
If the writing were of that caliber conceptually and carried out with equal excellence I would defend such decisions however Lorian chose to ultimately implement them whatever the restriction they might bring. This is the sort of specificity which would make the exceptions to my general rule. I don't mind a character whose romance is restricted to a single orientation providing there is justification for that beyond some arbitrary bias. No matter what shape the narrative may take, quality writing will always have my absolute devotion.