I would never get a tatoo. They just are not for me. I know many people with them and have no problem with what others do, but I doubt that I would ever want to be involved with a girl with more than a cute teddy bear on her ankle.
Only you know how much it means to you and if you can live with it. If she knew how strongly you felt, then it doesn't make sense that she would go ahead with getting two. Maybe she thought that you'd grow to like them.
If my wife were to come home with one, I'd still keep her. But I also know her well enough to know that that will never happen. She hates them.
So, what means more to you? The girl? The tat? Or the slap in the face that this seems to be? If you really care about her, then you owe it to yourself to talk over her motives. In the end, it's going to have to be your call. Telling you how we feel about it is like us telling you that you should like electrostats over Audio Notes. It's a very personal thing.
Talk it over with her. Take your time. Try not to make a decision before you've had a chance to calm down and think about it. Then, do what's right for you.