I got the impression Spawn-Astarion's enjoyment of sex is related to how he feels about his partner. When he is your friend he has a line about how his conquests failed to give him even temporary bliss often (something along those lines) and the drow scene doesn't go well with him either. Otoh hand he mentions that he overall enjoyed his time with the PC even though complicated feelings about his past soured that enjoyment. It's never clearly stated where he draws the line of "not wanting to be thought of in terms of sex" or how he eventually resolves it, but party banter suggests that once Caz is dead, he's at least heavily into pda again, to the point of annoying Lae'zel with it.
If he's your player character it's up for your interpretation how he deals with it, and thinking it through and coming to satisfying solutions for yourself is part of the fun. He can also always stay single and become a Harper.
Becoming a harper is a thing?
Generally I agree, that an origin as player character is up for the player to flesh out. If I would play one of them, then I would probably just go with what I headcanon would feel right, just like with every other character one plays in a rpg.
I saw people do pretty evil, tiefling grove raiding playthroughs with Karlach, romancing Minthara after, and having a blast. I just see it as alternate reality - if you play, Karlach can be evil, Astarion a paragon of good and Lae'zel open minded and engaging. Whatever is the most fun. I don't think, there is right or wrong in playing a character (with the exception that if it is a multiplayer, everyone should have fun and be on board for a direction )
