I think these are very good points, interesting to think about for sure. I can see him becoming infatuated with a party member or even falling in love with them. We can't know for sure how different it is for an undead but it's something to think about.
Yes, he does say he likes sex early on in the game, and it's certainly clear he enjoys it with Tav/Durge (if in a relationship) after he finishes his main quest. He also can potentially ask your character why you didn't invite him if you spend the night with Mizora.
It won't be any different for Astarion, as he's not “undead” in the classic sense of the word - the tadpole started up Astarion's organism again. The only things he still has in common with the classic vampire spawn are vampiric hunger, needing to drink blood, and also invulnerability to spells that work against the undead. But he can also eat normal food for pleasure, he breathes, he sweats, his body works the same as the living, except for bloodlust. Actually biting is his only ability as a spawn, he has no other vampire spawn abilities in DnD. Thinking about how the undead would feel only makes sense if Astarion doesn't Ascended and loses his tadpole, then he would be undead again with dulled senses.
As for the feelings of the undead in DnD in general, there can be different interpretations, largely depending on the DM. My DM, who is a big fan of necromancy and the undead, when I discussed the topic with him, said he thought Astarion's backstory wasn't realistic because the undead can't have sex, due to the fact that the body is effectively dead and functions differently. A vampire spawn could be a seducer, but it would be a seduction in words, flirting and such to lure a victim, but nothing more. Besides, there is no need for more than that to lure a victim. In this case, Astarion would have only wanted sex after the tadpole had revitalized his system. In terms of emotions and feelings - intelligent undead (vampires) can experience emotional and intellectual attachment, there are no limitations here, except that the vampire spawn have their senses dulled in general. But Larian has a different view in this case, and the DM who does Ravenloft campaigns also thinks otherwise.
