Thanks for all your help guys.
As you mention it may be a conceptual thing. The analogy fro BigGuy sounds good, it is kind of easy to understand and I might use it the next time this problem arises.
I've tried with some other analogies but perhaps they were not well explained. I tried to tell her that finding other girls attractive was like finding chocolate tasty, something normal and that is not up to us. She just told me that it was different and that the analogy made no sense.
I wonder if there is not a side of the coin to which I'm also blind. To me what she wants is only possible when one is so focused and absorbed on the other person that one develops tunnel vision. I don't think that that that is healthy but perhaps I don't understand what she has been trying to tell me.
I guess like you say it is going to be a long dance but your comments have really helped me. It is really nice to find this forum, I've discovered a lot of new things. Anyway, I'll keep trying to explain, I hope that we will be able to understand each other better and have a nice sweet ride.
I've agreed to be in a monogamous relationship. It would be nice to be have casual sex with other girls and I've fantasized with that but probably it is not worth the trouble and I'll certainly not cheat on her. I see this as a compromise and it is not a deal breaker.
She is not religious and she has a brother but her family is a bit conservative... well probably a standard family in Latin America. Anyway, she is about to move to Germany so I guess a change in environment will also expose her to new ideas.