Dude... She was testing you and you failed. If you really like her, you're going to have to do some serious work to get over this one.
On the flip side, there is something to be said for a woman who feels the need to test you like that. By doing that, she shows that she didn't have total faith/trust in you to do the right thing. Of course, you cannot bring that up now because , hindsight being 20/20, she'll just point out what you did and say that she was obviously justified for not trusting you.
But that answer really dodges the main issue, doesn't it? Had you even noticed this girl before she brought this up? Did you even care what she thought of you? If not, then your girlfriend's Eve and you're Adam. Sure, you shouldn't have eaten the apple, but she also sure as hell shouldn't have put it in front of your face in the first place.
My wife does not
play these sorts of games with me. She doesn't do that typically-female thing where they exaggerate what you say and jump off the cliff of logic to turn an innocent statement into an insult about her appearance or manner. And that's why she's my wife.
Honestly, here's what I think you should do. If you like her and want to stay with her, you need to do something dramatic to show her that you only want her. Once you've done that, you have
to have a serious conversation with her and tell her that you didn't appreciate her setting you up like that. Tell her that you never noticed that girl before and you would have never even had a thought like that in your head if she hadn't gone and put that out there.
If she blows up about that, she's not the one dude. Seriously. If she takes your comments and understands what you are saying, then be glad. More women will do the former than the latter.