As someone who is acepec, the whole demi Lucanis debacle just doesn't sit right with me. He has so many dialogues that just cut off suddenly, scenes that feel like they're missing (that missive in Act 2 where he talks about dealing with Spite once and for all that just became a completely different scene with Zara??) All the concept art for the romance that was never explored. The massive difference in relationship progression between Rookanis and Nevecanis in terms of both emotional and physical intimacy.
Lucanis' romance is so clearly incomplete even in a game where the romance arcs are sparse on the whole. His character without the romance feels incomplete as well - we get two or so big moments of characterisation and everything else is just the same scene about coffee repeated as infinitum.
I have no issue with a character being awkward and inexperienced, wanting to take things slow, not recognising flirtation or romantic cues, wanting to figure out their complicated situation before jumping into something. I am that person! It took 6 months after my partner and I acknowledged we had feelings for each other for us to start dating because I was not in the right place mentally to do so. Both of us are demi. Both of us have had very few previous experiences with romantic relationships. But that doesn't preclude a person from understanding or wanting emotional intimacy.
The Lucanis situation does not come across as 'he just doesn't have sexual attraction to Rook yet', it's just complete shutdown of conversation on all fronts (something he does not do with Neve, btw.) I too find flirting scary from people I don't know and agonised over it before my partner and I were together. This is not how he's written. His expressions do not reflect shock or awkwardness when Rook flirts, nor does his tone when he responds. Do you know when he does do this? With Neve right at the start of their banter. And he overcomes that shyness quickly. He just does not acknowledge Rook speaking at all the same way any video game character does when their conversation tree is bugged, the game decided to cut some response dialogue and just transitioned directly into the neutral follow-up line. He isn't rejecting emotional intimacy because of Spite and the Ossuary prison because during the ambient dialogue it's clear he's sharing that emotional intimacy with Neve waaaaay before Rook helps him.
I just want more of his writing. Where is he and his character in this story? He's so incredibly inaccessible to us. And I know people will say that characters do not have to be emotionally available to the player (which is true! A great example of this working well is Vivienne, who is warm but not directly open if befriended.) But that should then be a consistent character trait, and it's clearly not. In fact, Lucanis is incredibly declarative about his feelings for Rook romanced or not once you hit Act 3, he's just completely missing in Act 2. Which to me is a flaw in the game's structure. And that 'missing-ness' is being read as demisexuality instead of 'hey this character's writing is actually incomplete and needs restructuring and additional scenes.' Those additional scenes could very much be as awkward and non-romantic in tone as you please, but we needed more of them. Have Lucanis visit Treviso more often to do things other than drink coffee. Have him visit other locations as well! Teach Rook how to cook a local dish, the gondola fight we saw in the concept art. Deciding after the game's poor reception that a character who is clearly only 3/4s of a character, who is lacking in so much characterisation, is the demi character? That feels bad man. Demi characters should be written with just as much care and attention, should get to express themselves in as wide a variety of circumstances, show us more of themselves and how they interact with the world. The reality is that Lucanis has the weakest romance arc in the game, and posthumously attributing that to demisexuality feels like an indictment of acespec people's ability to read the room and express emotional intimacy rather than a true attempt at representing us.
Just as Cullen's heterosexuality and preference for elves and humans is in fact a game development constraint rather than a character writing decision, and Solas' heterosexuality was a result of running out of dev time to record voice lines and re-rig cutscenes, I think we should be able to acknowledge that this version of Lucanis is not a good representation of his character in multiple ways. And that is due primarily to the development cycle rather than writerly intention.
We've had beautiful slowburn romances that were also deeply affected by character trauma throughout the Bioware series. Lucanis' is (in its present state) not one of them. Leliana, Cullen, even Alistair. I think using demisexuality or even slowburn as an 'excuse' for Lucanis' incomplete character isn't a useful way to think about his writing.
Note: This post is purely about quality of writing wrt emotional intimacy. I have zero issue with virgin Lucanis and think that fits his wider writing. I also don't wish Lucanis to be someone he's not - Zevran or Davrin or some other flirty/dommy archetype. I just don't like the casual conflation of awkwardness and inexperience with asexuality. These things do go together but also I don't think we should use demisexuality as a bandaid for poor writing because it reflects poorly on our understanding of ourselves and the community.