It's pretty simple: accept your gf for who she is, or leave because you would rather be with someone else. Both of these are valid life choices, its only if you try to do both that it becomes wrong. Leaving would be kind of shallow, and you might regret it, but relationships are voluntary.

I would examine the contradiction between "I love her" and "I really wanna fuck this other girl." One of those is false. (I am not talking about poly people, they exist -- but they enter relationships with that expectation not peek over the fence for the ass on the other side.)