Sounds like she's being irrational. Not much you can do about that kind of mindset. If she had a legit beef about it -- legal issues, concerns for your health, etc -- you'd have something to discuss or even negotiate. But "it feels the same as if you were cheating on me" -- ???? HUH?!?! There's no correlation between your behavior and her feelings. None. And that's not your fault. Pick some random behavior of hers and try using the same argument. "I don't like it when you get a new pair of shoes, because...
it makes me feel like you're cheating on me!! Isn't that ridiculous? The one thing has nothing to do with the other.
Wimenz...

theyz nutz.