Yes, I see what you mean, and I agree. I don't think that Astarion is ace either, but I'm thinking more of Tav here.
What I mean is that if my Tav is ace/demi, there's no route to take here. All that matters in the relationships in the game is sex, more or less.
Apparently there's no intimate scene with Wyll, so, I suppose he might be a better choice for this, but I don't really like poor Wyll so I haven't tried that romance yet, and therefore can't say anything about it (yet).
But, what I was missing in the game was an option where I as a player was able to romance someone without it focusing on physical intimacy. And, that's why I found Astarion to be somewhat of a good choice considering he clearly needs time to deal with his SA trauma and he also needs to have a deep emotional bond to want to have sex (good route here obv.). This means that Astarion, at the time, sort of comes across as somewhat demisexual (even if it's trauma induced).
So having the option to romance him without the sex, and instead focusing more on emotional connection, would cater to ace/demi players without ruining anything about Astarion's character. I mean, he even acknowledge the fact himself that he and Tav "haven't...in a while" when you talk to him about Halsin coming on to you etc. So they are already sort of having a relationship without sex, it wouldn't be a stretch to keep it that way for a little longer. As much as I like the graveyard scene for example, there could have been an option there to just hold him.
So having the option with Astarion to be close without sex would in my opinion be a decent compromise to represent ace/demi when there's no chance at all of having a new romance option that would fit this role.
Edit to clarify: Now, sure, at some point Astarion will probably get back to his old self and will want physical intimacy in a way that would probably not suit an ace Tav. But I mean, in my world that would happen after the game, possibly long after, and whatever happened after the end isn't really something I tend to bother myself with. Maybe they break up after a couple of years or something, what do I know. I just don't want to force my Tav to sleep with the one she loves in order to keep them as a partner. Sex isn't everything that makes it a romantic relationship (at least not for some)
