I don't think it's a case of people trying to pretend he's a good guy, for much of the stuff you said. He's still a largely immoral, egocentric sister-banging asshole who's done pretty much one redeeming thing in his entire life that he can't even talk about. I think the main issue people have is what I kind of outlined, which is it was just so damn jarring to how him and Cersei are portrayed in the books. Cersei to Jaime is everything and I can't really picture him deliberately being sexually violent towards her, at least not to THAT degree, you know? Just because he's guilty of a list of immoral things longer than my arm doesn't mean it's in his nature to do absolutely everything abhorrent, you know? It's kind of like if suddenly, Sandor Clegane turned around and raped Sansa or Arya. He's a cold blooded killer and a violent, absolutely unlikeable man (well, one of my favorite characters, but if we're speaking from an in-series perspective), but I can't imagine him ever doing anything like that, especially since his brother is the reigning champion of wartime atrocities. Or if Jon Snow turned around and started bullying Sam Tarly to get the approval of his fellow Brothers. 

It's not that people are really offended by the act itself, because you kind of have to be used to it/ accept it as a part of the narrative to stick with it this far, it's just that how the scene in question was executed was a huge departure from the book and what people feel is Jaime's character. If they played it out more like the book, I don't think people would nearly mind as much. Keep in mind that a big part of Jaime and Cersei's narrative moving forward is how it becomes more and more clear that Cersei never loved Jaime, at least not to the extent he loved her, and was more or less using him, and him starting to discover that fact and distance himself from her, which becomes quite pivotal later on. He's proven to be entirely devoted to Cersei, and there's been nobody else, something he's pointed out on a few occasions. Just how that scene was executed just felt plain wrong and at odds with the book and blah blah blah, we're going around in circles. :P

TL;DR Jaime's a terrible human being, but how that scene played out really didn't seem to reflect the character properly, especially compared to how it played out in the source material. He's capable of a great many crimes he needs to be held accountable for, but what was widely interpreted as a passionate act between two receptive parties who loved each other since birth should have never been portrayed as a cold, violent rape for reasons we've beat to death kind of like what happens later on in this season.