It's not only about her masturbation scene, it's about the entire pacing and engagement.
I can't speak for dontcarewhateverno, but at least for me, it would be a lot more engaging if there was some sense of romantic/relational progression. I don't mean this in the AVN vernacular, i.e. filling up buckets, but the more realistic push-pull dynamic of real world relationships (both with MC and the antagonist). I just played it again to make sure I'm not misrepresenting it, and indeed, there's almost none of this back and forth, the proverbial flirting dance (or scorn or any emotion). Where's the push-pull? There's almost no apprehension or tension, it's like watching characters in the Sims do things solely because the player has directed them to do so.
Now, she is from a different time/world, so it could be chalked up to culture shock. At the start, her relationship with MC feels oddly platonic for an AVN. Then the antagonist enters, and within a few scenes Celeste has these internal monologues hinting at sexual excitement. This feels like a juxtaposition; I'd expect a person not wary of strangers to behave similarly in either circumstance.
This is part of why I generally dont like bbc fetish focused ntr games. They often use that fetish as a catch-all stand in for relational progression. Cedric isn't particularly charming or good looking, but that doesn't seem to really matter to the narrative.
You could take the same characters and write a much hotter story with Celeste initially being standoffish towards the antagonist then slowly coming around, OR one where the antagonist started off as a silver tongued devil, playing the charmer around both Celeste and the mc- then at some point MC realizes things are not what they seem.
