I think this is one of the issues here. Separate from the over-the-top tumblr-esque easily outraged folk, just looking at it in the context of the show it seemed particularly gratuitous. Yes, there have been untold amounts of horrible, dark, twisted things happening in the world of the show. But, for the most part, I'd argue that they usually were doing either one of two things: 1. staying true to the source material (as far as i know, I only go off what the actual book readers have said) or 2. Furthering the story.
This scene falls into a bizarre grey area. By all accounts, this doesn't occur in the books. Fine. We've practically caught up with the source material now and increased divergences were inevitable. But did it further the story? Not in my eyes. We were given a season of Ramsay's depravity with the constant torture scenes. It's been beyond well-established that he is one of the more, if not the most, despicable character in the entire story. Was this a necessary addition to the show? One might argue that it's depicting how, despite his words to Littlefinger about not hurting her, he's going to treat Sansa. But you could equally argue that that was the entire point of the scene at the dinner table when he suggests that Theon give her away at the wedding.
I'm not offended at the rape or its depiction. But I do question its necessity. It just seemed gratuitous to me.