When is sex appropriate in a horror story? When is it not gratuitous? Must it advance the plot or is it enough that it aids in character development? If it doesn't do a damn thing for character development is it still appropriate to throw it in there?
I may seem like the last person to answer this question. I have occassionally been accused of gratuitousness in regards to the sexual content in my writing, though I would argue that I have never written a single sex scene that hasn't either advanced the plot or helped define the characters involved. If anything, I write a lot of plots that revolve around sex but plot-driven sex is not a crime. I think I'll Twitter that.
So when is sex appropriate in a horror story? When it would be appropriate in life, which is to say anytime and any place where children are not present and you are not in any imminent danger. That's my answer. Short and sweet. I'm sorry if you were expecting some long drawn out essay but I just don't feel it's necessary.
Art should imitate life as much as possible and in life you need no justification for having sex other than a mutual attraction. I'm not suggesting that everything that happens in life should go into your novel. The daily minutia of life needn't be included. You don't need to know every time someone passes gas or picks their nose. But sex? That's a bit too big to ignore and how a person makes love says as much about who they are as where they were born or how they were raised. Who they make love with tends to be important to the plot. If someone has sex with the main antagonist of the story how could that not be important?
So, in my mind, sex is always appropriate where it would not seem ridiculous. When it is natural, it is right and looking at the broad spectrum of human sexual behavior, both moral and amoral, I would challenge anyone to find a scenario that would be "unnatural". People fuck. Deal with it.