It all depends on the person. Some people choose to save it for their own moral or ethical reasons. Others choose to do what they want, and just enjoy the moment. And there is nothing wrong with either choice, as long as two consenting adults who are making the decision.
EmperorNero: You really have to re-evaluate your situation with your girlfriend. If you have such a major hang up over her making a mistake, then why are you with her? I am sure that she regrets enough her past errors, with out having to hear it from somebody else. If you cannot stand the thought of what she did, then that is your choice. But she cannot change it, and being with her when you feel this way, is only going to hurt you both in the long run. And I am not bashing your life choices by any means, if that is the way you want to be, then good for you! But getting mad at her over what she cannot fix is just not right. I lost my virginity when I was only 15 to a 21 yr old, and it was the worst mistake I ever made in my life. But I moved and learned from it, partly because I had supportive people in my life, and partly because I realized I was young and stupid. And I don't think I am a bad person for it, instead I think it helped me grow up, and realize what was important in life.