If nothing else Zed answered the age old question, "Does he look like a bitch?"

How about Irreversible? If that's not canonical horror it still meets my personal definition. Instead of the usual tropes, it actually tries, effectively imo, to induce nausea with it's camera work and disconfort with the reverse chronological cuts. The brutal rape is fundamental to the story, connecting as it does to the themes of reproduction and futility.

I'm not going to get into a "Yes" or "No" because... seriously?

Let's look at what horror is and why it works though. The most popular horror stories are the ones that have been ground in reality in some form, rooted to "What if" and "Could it happen?". The earliest examples you can point to involve the works of Alfred Hitchcock. The attack of animals against humans en mass in a way we can't defend against, a man who has his ability to connect between who he is, and an important figure in his life, the story of a man who struggles with the knowledge of a neighbor who is violent and able to hide his activities. Hitchcock worked because these were themes that people could see happening, and could genuinely be concerned with happening. This was an era when there was so much unknown detail in medicine, mental disorder and during troubling periods of people bringing about fear of murder or harm.

This can further apply to something like the Texas Chainsaw Massacre, a movie heavily based on a real event, again dealing with mental issues. And everything was framed as to make it much more real, and brought a fear of strangers and the ill.

When we get to the 70's and 80's, we started looking at space, computers. Nobody was sure how computers could advance, if they could become so smart that they would turn against us. Novels like "I have no Mouth and I Must Scream", movies like "2001: A Space Odyssey" and "Alien", they were the fear of unknown, would could be in space and what we could have created.

Now, as far as the asked topic in question, violence against one another is always a possibility. It's looked at from a very, very similar perspective in every situation in media, generally sexual violence against women, in rare instance by demon or monster (See the Bruce Campbell films, the Evil Dead series as one example). Rarely does movie or film focus accurately on the situation in a way to actually pass that intent to horrify or show brutality of it towards the victim, instead either turning the situation into a means of revenge/vengeance, killing the victim soon after or by having them simply "get over it" in a brief period, when in actuality the event causes a large amount of various harmful effects physically, emotionally and psychologically. One example I would point to as being a rarely seen example are two characters from Fallout: New Vegas and the effects that it has on the characters around them (Granted, being a videogame you "solve" the problem fairly easily, so it's still not the most realistic situation). Another thing I would point to is actually the film "Gone Girl", where the premise of multiple sexual assaults is turned on it's head, the perceived victim for the entire film in reality orchestrating the situations for her own benefit and taking advantage of the situations for her own benefits, no matter how brutal the means or methods (And men, should you watch it, will probably be fairly horrified from sex for a while.)

In any case, the eventual answer is that as a theme of horror it's an item that can be used to some effect, but you would not put zombies in every horror story. You would not put zombies in every horror story, nor aliens, sentient robots, chainsaws, cannibals, Hannibal Lector, Genghis Khan, evil Jellyfish, shapeshifting clowns, tentacles, octopus, lasers, flamethrowers, robots, Steven King, werewolves, skin-suits, flesh-covered books, witches, dark magic, Satan, occultists, spiders, cockroaches, birds, strangers, or fear itself, in every story. They might try to use it, but so rarely will they treat it in a way that has any real impact and doesn't take away from the seriousness of the issue.

"And Marcellus Wallace don't like to be fucked by anybody except Mrs. Wallace. "
Pretty clever foreshadowing, that.

I know about the movie, but haven't watched it myself, and didn't feel comfortable using something as an example when I only know it from hearsay. But it does seem to be another good example, especially since it seems to have a higher artistic value than the movies a mentioned. Gaspar Noe certainly is a more competent director than Meir Zarchi, the director of "I Spit On Your Grave".

If rape is a major plot point that explains why things are being so horrific to the humans in the movie then sure.

This east Asian movie below has a rape scene which explains the suffering of the main characters.

https://www.youtube.com/watch?v=vsRcK2dOQz4

I've heard it called "the Twilight Saga for adults". I wouldn't know, I saw/read neither. My female work colleagues certainly seem to dig it, judging by how much they talked about it when it came out (and most are reading the novels too).

That blew my mind. Its like figuring out it was Bruce Willis who keyed Vincents car.

Apparently, it's just porn, like those typical "romance" novels.
The original "novel" has a rather bizarre backstory, though: It started out as poorly written "Twilight" fan fiction on the internet; and when it got picked up by a publisher, the author removed it from its website, did a find/replace on all the Twilight names and thus turned it into a poorly written bestseller.

Why not? We have movies and media featuring mass killings, PTSD-inducing battles and other such things. Why is talking about "rape" such a controversial subject? It happens. We need to talk about it, and get it into the public consciousness that rape is bad and the only way to do that is to push it into the forefront, so people ARE talking about it. Hiding and not talking about something is a bad way of dealing with things.

I hate to reply just out of pedantry, but Syliva Likens was not kidnapped. The atrocities committed against her were by the woman her parents paid to watch her, as well as by her kids and a neighbor. As I recall, her parents worked in a traveling circus. You are correct. though, that the rape in The Girl Next Door was an embellishment. For a good nonfiction account of this House of Evil: The Indiana Torture Slaying was republished a few years ago.

I won't dignify the rest of the argument with a response except to say that the entire appeal of Jack Ketchum's novels is that he doesn't fall back on the supernatural to horrify his readers. Mr. Ketchum has realized that man is capable of enough horrendous acts that demons and monsters aren't even necessary to scare people. "Evil", in as much as it may or may not exist, can manifest itself in anyone and that is more terrifying than any boogeyman found in the works of mainstream horror.

By the by, at least three films centered around Unit 731 have been released.

You're now just cherry-picking what is OK to laugh at and what isn't. Here is a video for you:
https://www.youtube.com/watch?v=fwMukKqx-Os

But again, you didn't answer the question, you just threw up smoke: how does depicting something trivialise it? Is anyone going to watch a horror movie and think "oh well, that demon-posessed maniac just raped someone, I guess that's just how life goes". No one thinks like that. We know it's horrible, that's what people watch horror movies for.

Saying "only mentally shaken" you might as well understate the severity of it. Many people suffer a lot from mental problems. Just because you cannot see it easily doesn't mean it doesn't hurt really bad.

But anyway this is not the problem, is it? Noone dies or is mentally shaken while making or watching a horror movie. It's just a result of fiction and in principle people should be able to differentiate between reality (violence, brutatility, rape, murder, ... is all forbidden and evil and a crime) and fiction.

Frankly, I do not watch horror movies and find it hard to understand why anyone would like them. But then if some people like them, let them be as long they restrict their desires to the fictional world. That is probably the crucial part.

And additionally we have to protect our youth from being exposed to violence, brutatility, torture, .... That is also crucial.

Probably we could learn much about us if we would understand what horror (or rape) stories give to some people.

My two cents:
Of course the biggest elephant in the room is "Phantasmagoria". If you haven't played it, you should. It is one of those games that you have to play just to... broaden your horizons, let's say.
There was a rape scene, but it was done very subtly, I didn't even know that I was seeing a rape scene until half way through (or maybe I'm just very stupid).
In my opinion, everything is allowed as long as it adds to the experience and makes sense.
For example: you cannot show the life of a street gang without street fights. Imagine what "Green Street Hooligans" would be without all the violence.
The same applies to horror movies/games/books/etc. If you, as a creator, absolutely sure that this scene would be strong and leave some trail, then you are free to do what you desire, BUT - it should add to the experience and make sense.

I have been developing my own story for a movie that I want to shoot with my friends and my vision actually includes a rape scene. It is not a horror movie, it's more of a social drama.

"The main hero is full of hatred, anxiety and he tries to socialize in a small town.At one point in the movie he develops romantic interest to a girl, then he starts to spend some time with her and evetually finds out that she just friendzoned him and his phone number is marked as "Free Food" in her phone. And so he rapes her......
Overall, the main theme is "Hatred/Hate" towards the humanity and what it had become.
I don't know, maybe I'm just insane in the brain and a horrible person, but it was very hard to share that piece with you guys.

Anyway, yeah, all the fragments must create a vision and add to it. If the creator will pull all the strings and create really hard hitting experience, then it won't matter if she/he included gore or rape or bestiality.
Just don't roll into pure madness, it's very easy to do.