You very early on get the chance to find out that he is just spinning you along and all he sweet words are lies. You can then either warn him off or go along with it for the fun and sexy time because you think he is hot.
When in the first act before the romance scene can this be seen? On first playthrough, the beginning of the romance looks exactly like Astarion's sudden burst of desire. Of course, it's silly to assume that such a "trust no one" has fallen in love, but that he wanted intimacy with Tav is more than believable, and it feels that way. Relationships can start with intimacy, and you can expect to earn trust and grow closer to a person over time, gradually melting the ice, proving by deed that you can be trusted. Tav gives in to passion and finally falls in love with Astarion, after all Tav could die at any moment, there is no telling what will happen to the larvae, and to give up such happiness for even one night seems strange to me. The strong desire to talk and find out what's going on with him, instead of immediately falling in love, comes when they change his facial expressions and make an unhappy face, while really not adding the appropriate lines and reactions. I'm glad I went through this scene for the first time back when he didn't have that unhappy facial expression and I was able to really fall for Astarion. And in my headcanon I still hope he enjoyed it at least a little bit too, he called Tav beautiful and when I put my neck up for him to bite, he looked very predator-satisfied. And I don't think this chaotic fire that erupts in response to his advances relegates me and my character to the simple platitude of "have fun and have a sexy time", it's more than that.
Later on you can ask him if Cazador's victims really fell for his lame routine, for which he mocks you because you after all fell for it too. And why did you fall for it?
Astarion has never mocked me (except for the fake sadistic Astarion in patch 6), those lines I don't choose, but it's good to have them, it allows you to reveal more of him, I see his manner of seduction as charming and (pardon the tautology) very seductive. In the first act, I'm completely fine with the range of emotions, right down to the lines and actions I would have done myself if I were Tav. The only thing that spoils the roleplay is the inability after this scene to openly announce to everyone that I'm dating Astarion, here I have to turn on the headcanon to somehow explain this strange need to hide the relationship. In the scene after the ritual, when I already know about Astarion's injury, when I'm more than serious about him, I would never think of such dope as: "I want your body." And I don't know of anyone to whom it would have come at all. It's primitive, it doesn't reflect the significance of the event, it doesn't reflect Tav's attitude towards Astarion. And most importantly, it breaks the classic rule of dialog in RPGs - when we have conventionally "good" (altruistic, loving), neutral and negative lines. The first act scene has this classic set, the post-Ascension scene has all the lines being negative to one degree or another. And there's a difference between there being no way to start the romance any other way than to give yourself to Astarion (after all, it is a romance, and BG3 itself is just overflowing with sex) and between looking like a horny nymphomaniac when the romance is already deep and serious. That's just as surprising.
Yeah, I personally have always really, really disliked his act 1 romance scene. I think it's just a me thing, I'm going to spoiler it since it's quite negative:
even back in Early Access when I didn't know other fans I just had to mash the spacebar every time to skip through it because it just made me cringe a lot, I couldn't listen to it whatsoever. My personal opinion is that it fails at being sexy and is instead a quite bad impression of what a seductive person talks like, not to mention very obviously fake both in the lines themselves and, honestly, the acting, I think. I'm not sure if even back then he had the instructions that this was all completely an act since we know that it was another writer who brought that angle after EA, but it kinda feels like they had some idea (because of devnotes like "winning at seduction") because it truly is too on the nose. At least the part where he doesn't talk is actually pretty good, but whenever he's talking it's terrible for me, but I had to do it because I really liked Astarion and wanted the scenes that came afterwards, like the scar reading which I love.
For actual sexy scenes, I'd say both good and evil Shadowheart's act 3 scenes are pretty sexy, and parts of AA's scene except when he's savouring that you kneeled because he lays it on really thick and does some really weird poses.
I played after the release, and I missed Astarion's seductive lines in response to the phrase "You don't have me yet" (if I understood you correctly that you don't like these lines) because I don't like Tav's line, it seemed to me too much like a possible rejection, I was afraid to push Astarion away (flirting is not my strong point, that's for sure :)) and immediately went for a kiss, without words. I played it without spoilers, completely unaware of what was going to happen next or where it was going to lead, and just opened my mouth when I saw his scars at the moment of the bite. Astarion was walking around in his white shirt at the time, never changed his clothes and I actually saw his scars for the first time. That whole scene really impressed me, just as much as the scene in the second act impressed me (although there were different feelings there and it's hard to compare them). But Astarion's lines, when I watched them on video, I found very seductive and charming, just as I found Neil's acting to be great, but of course everyone has different perceptions and feelings about one or the other. But the dialog scene after the Ascension gave such a negative contrast to the previous two romantic scenes that it completely breaks the immersion in that moment and instead of "being inside" and feeling and perceiving what is happening, one wonders and asks, "Who wrote this at all?". I believe that when a player goes into "critic mode," that is, the player begins to evaluate the quality of the script in a negative way, right at the moment of play, detaching from the game, withdrawing from it emotionally and mentally, this is the clearest sign of a poorly written scene. And I agree with you about the scar reading scene, that scene just cuts to the heart.
Back to the topic, I'm pretty sure I've brought this up in the other romance improvements thread, but I'm actually quite partial to the approach where Tav is there to serve as a narrative tool rather than being, let's say, more roleplay focused, because I don't care so much about roleplaying a character in contrast to experiencing an interesting narrative and the world the devs have made for me (I will say I was a bit confused I could only be mean to Naaber, lol.). But this is very much a personal preference, there's definitely no harm in adding an extra option that is after all still going to lead to the same dialogue branches that are already ingame.
I know what you mean, although I have rather the opposite approach, I really appreciate roleplay and the ability to influence events rather than be a tool, which is why RPGs are my favorite genre. The option for the player to observe the characters and story "from the outside", without involving themselves in the plot, fits much more with, say, an interactive novel than with a genre like RPG. And when it's claimed to be an RPG, the player feels cheated. I don't mean it's so important to express directly my character, but interactions with other characters (especially the dearest character) should feel real, not someone else's story, at least so that there isn't such a stark contrast, Tav shouldn't be the opposite of the player and what the player represents. If the basic principles of dialog construction in RPGs (conditionally positive, neutral and negative lines) are observed, there will be no such a knock-out contrast. The author of a book or short story can be guided only by his own vision of the story, but the author of RPG must necessarily take into account the player and the possibility of roleplay, it is a feature of the genre. A novella can be liked or disliked, it's a matter of taste, but an RPG is always about roleplay, and when, buying an RPG, the player gets into pieces of the novella that he doesn't like, the RPG ceases to be a good RPG, at least in those moments.
In addition, the lines for Tav in the scene after the Ascension are also extremely selfish. "Become a vampire" is a selfish line, as if Tav sees Astarion as a tool to fulfill his own needs. And the "body" is the same way. Astarion becomes a tool for Tav, Tav becomes a tool for the narrative, the player's hand becomes a tool to make a facepalm. Alas, gentlemen and ladies, it's low quality scripting when tools stick out of every crevice of the narrative. It's like an irresponsible attitude towards plot and narrative logic amidst a lack of understanding and/or unwillingness to do "dark romances".