bless OP for putting into words what i've been trying to articulate. when my brain is in the Writing Place, i end up writing a lot of smut and... horror-adjacent stuff? psychological horror? i read and write darkfic, is what i'm getting at, and i do consider what i look for and what i make for myself to be in that genre. i don't tend to go in for particularly bloody or gory works just because too much of that is my squick, but mild body horror and monsters and psychological stuff gets my attention, especially when you get into themes of perception of reality, autonomy, social engineering, that sort of thing.
having read/written smut and horror separately and together, they're sister genres. the umph that you get from psychological or body horror is made of the same building blocks as effective, filthy smut. my sex scenes have improved as i've written darker stories.
both subjects are about intimacy, what you can do with it, what people are like without it or when it's withheld from them, either between characters or a character's intimacy with themselves. it's the same reason why, say, a lot of scenes of violence in non-horror properties end up feeling sexy or why villain shipping is so so popular.
also, it can't be left unsaid... the physical act of sex is gross (positive, in this context, all of this is positive). it's squishy, awkward, and strange. horror doesn't tend to shy away from the inherent awkward squishiness of the body or the sensitivity and strangeness of the mind and has made a whole genre out of effectively leveraging them instead. writing good porn is the same, just with some of the dials adjusted. sex is already kind of a gross weird thing to be doing, that's kind of part of the point. if you're in the market for writing explicit smut, leaning into that is the best way to make it filthier and more effective.