There's a lot of way to do horror. My favorite horror stories are things like Alien and The Thing, with physical suits and miniatures, characters using their intelligence to keep themselves alive instead of sheer luck, plenty of mystery as to why the scary thing is there. Other people like stuff with lots of gore like Saw or jumpscares (like....everything Hollywood does nowadays, and most video games). It Follows is super good as a horror movie that I think could also work really well as a written story, too, because the scary thing has rules that it follows and there's ways for the characters to think of ways to stay alive...but ultimately it won't stop. The only way to live is to foist the monster onto someone else. It's scary because there's a monster that's going to get you, but also it requires you to make a monstrous decision to stay alive (which you see in The Thing, too).
Mixing it with erotica is interesting. Again, It Follows is ultimately about sex, so when the characters have sex, there's (spooky) plot reasons for it to happen, not just porn without plot. Sex (at least to me) is ultimately an act of trust, you're lowering all your shields and putting your trust into someone else as you put yourself in a position to be hurt very badly (physically, emotionally and mentally). So passing on a curse via sex can lead to some interesting inner thoughts/dialogue (which you can't have via a movie but can in a story), and it's extra scary because someone doing that to you is such a huge violation of trust. You could also do something like Teeth.
I should probably get around to joining that subforum this stuff sounds fun.