Is she willing to watch any porn at all with you? If so, does she have a girl that she likes to see? Maybe she could watch a scene where the girl is obviously enjoying anal sex.
Have you asked your girlfriend calmly what it is about anal sex that makes her not want to try it? It's important to ask this in a non-pressure situation, not when you're already starting to fuck. That way she can answer it thoughtfully. Is she afraid it will hurt? Is she afraid it will be messy?
Finally, if you haven't been insisting on anal, how do you know that she won't give it up? ;)
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