Okay. :) (Fair warning: This post necessarily contains spoilers about the two movies.)
First movie is the aforementioned
Deadgirl (2008). It's a movie about two teenage boys, outsiders with no friends, who discover a naked zombie girl that's tied down on a table in an abandoned facility. One of the boys sees her as an object to fulfill his fantasies with; this includes repeatedly having sex with her although she fights against it. The other boy sees her as a person or at least as a creature he can empathize with, and he's torn between loyalty to his only friend, temptation, empathy with the girl, and his conscience.
Why do I think that the movie would not be better off without the rape scenes? Because the objectification of the girl is a huge topic in the movie, and rape scenes are the ones where this becomes most apparent. The movie deals with the question "How far would boys/men go if given the opportunity to do anything they might want to a defenseless girl, without fear of repercussions?". This obviously makes rape a topic. Excluding it from the movie would have left an artificial gap, because everyone knows that in real life there obviously _are_ men who _would_ rape the girl in such a situation.
I like this movie because I think it depicts an important topic in a surprisingly well-done way. I also like how it transcends the clichés that often limit the horror genre. In this movie, the zombie is not the dangerous monster - despite behaving more animalistic than human, she is the character that viewers tend to feel the most sympathy with. That was a welcome change to regular zombie movies (which I often find very boring, because most never even try to break out of the genre's very narrow formula).
Was this movie "fun"? No, of course not. But that was exactly why I consider it a good movie. It stays clear of the (imho) boring, mindless gore and cheap scares that a large part of the zombie film audience apparently considers "fun", and instead provoked thoughts and discussions about the human nature.
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The second movie is
I Spit On Your Grave - the original from 1978, I haven't seen the 2010 remake. It's a "rape and revenge" movie with a very crude plot: A female writer is alone on vacation, gets raped by four men, and then kills them gruesomely one after the other.
This movie contains a 25-minute rape scene. (Edit: Some sources speak of 45 minutes; it depends on whether they talk about the original version or a cut one, and whether one sees the consecutive rapes as a single scene or not.) This caused _lots_ of controversy, which is understandable, because people automatically assume that there can be only one reason for such a scene, "shock value". And adding an extensive, detailed rape scene just for shock value would be - I agree with that - just another objectification of the victim.
However, if you actually _see_ the movie, you find out that it's quite different. The entire rape scene is filmed in a way that you identify with the victim, and it is not embellished with anything. It is a brutally realistic depiction of a rape how it happens every day at some place in this world. The male audience that went into the theaters because they thought it would be cool to watch "the rape movie" ... was treated with a 25-minute experience that let them feel the entire humiliation, brutality, and hopelessness of a very realistic rape from the victim's perspective; an experience that men very rarely try to empathize with, let alone for 25 minutes.
Was this movie "fun"? Hell no. It was _meant_ to be disturbing and unsettling. The director made this movie because he had experienced how a woman was raped and then everyone closed their eyes about it, and he wanted to rip those closed eyes open. And if just a few loudmouths who went into the movie because they thought watching a rape scene was "cool", ended up unsettled and disturbed, then it was certainly worth it.
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So, here we have two movies which are horror movies, which have rape scenes, and which would not have been better without those. (In fact, in case of "I Spit On Your Grave", the rape scene is probably the only thing that elevates an otherwise very forgettable movie over the exploitative "rape and revenge" dross.)
Did this help you to understand how the concepts of "horror" and "rape" - imho - can work together?