I have a similar problem... many of my fantasies involve abuse or rape or the like (often a combination of it all). I have gotten to a point where I can force my body to just enjoy what is going on in the moment, but it's very difficult to do, and takes a lot of effort. Most of the time, I just launch into one of my stories in my head.
A therapist I saw at a trauma program has a blog, and just wrote on this topic the other day... Becoming Superman... I think it's 2 or 3 entries down. It was an interesting take on the topic of sex and abuse and norms...
It's very weird that I am seeing so much on the topic lately when I am struggling myself with questions around all this.
TRIGGER WARNING FOR THE FOLLOWING:
I started self-harming in a way to rid myself of the feeling of violation... it became an addiction, as the result ended up with quite a positive re-enforcer, wiping out the real impact of anything negative that happened as a result... My fantasies have always contained multiple elements of abuse and force, but this took it all to a different, weird, shameful level... I've never actually talked about it in this much detail before, and this is a bit weird to say the least. I'm fascinated that others go through similar struggles, even if it's just the weird fantasies... I have not found anyone in person that even remotely admits to anything like this. While I am sad that more people are going through it, I'm relieved that I am not totally alone... Now I just need to figure out how to talk about it with my new therapist...