This is most interesting. My view as a sadistic writer is that if you're going to be nasty, then BE nasty. So I guess you're reading the wrong stories. You might try my 'Agony of Simone' story in the BDSM library here, I try to include realism, but I write lesbian/animal rape, so it may not interest you. But that aside, I agree with you. But for those S&M writers you talk about, the 'aroused' victim is just their own fantasy...or maybe they are scared to go the whole way.
Thanks, I'm glad you brought this up, it gives me more confidence to write about only genuine abuse situations. And I'm all in favour of ring gags.