I can only speak from experience. Let it go. I decided to press that issue with a girlfriend once when was about 20, and that was the beginning of the end of that relationship. The funny thing is, as I got a little older and had a one night stand or two of my own, I totally got where she was coming from, and it's never bothered me since. Now I'm not saying go have a one night stand so you can get over this, in fact it's not a real healthy practice anyway. Just take my word for it, it isn't a big deal if someone has had one or two in their life, it just proves they are human, and the fact that she was honest and told you about it, should be an indicator to you about where you stand with her, right here, right now, when and where it realy counts. The past is the past, concentrate on the now, and life will be a lot easier.
Again, just my $0.02.
ViSionS
Problems cannot be solved at the same level of awareness in which they were created. --Albert Einstein