Well, damn. This is a pretty severe whiplash from the last writing piece I read (not by you two).
First of all, I have a deep appreciation for the characterization the two of you are capable of. I've read and been told that a good character should be visually recognizable even in silhouette. That's mainly from an artistic side of things, obviously. I think the written equivalent would be characters that are distinct even if they go unnamed. Ignis and Deni both have that quality. They're both filled with personality, idiosyncrasies, flaws, and virtues. Little things like Ignis becoming distracted by the mechanics behind every new thing she discovers and running themes like Deni relating most things back to his storied past while forcing a nonchalant facade, it makes the characters feel familiar even after time away from reading their stories.
Moving into the meat of the story itself, I enjoy the segue of sorts, where the violence was the catalyst for their behavior, as if it makes them more reckless -- a fact which makes sense in hindsight and which they've recognized in themselves. They were both reckless from the start, and being together has only exacerbated that. The detail and emotion in the love-making scene managed to be evocative without being obscene, but having read the Isvaskur arc, in the back of my mind there's a dread. I know it's wrong, Deni knows it's wrong, only Ignis is unaware. As much as I like Deni as a character, there's very much a, "This bastard!" vibe while reading it.
Even with that mounting feeling, it comes as a surprise when Deni confesses, and the way he does it is both shocking and in keeping with his character. Self-destructive and yet trying to justify something he cannot even justify to himself. Ignis' reaction is heartbreaking, it hits home as someone who's felt and seen that kind of betrayal. Fictional or not, it resonates.
In summary, damn. Just damn.