Ok I am gonna try not to get irrate about this as I have a real issue with people who continuously use someone's past as a Shitti stick to beat them with.

This is 100% your problem, not hers. So don't make it hers, just cut her loose and be done with it.
I know a hell of a lot about this kind of obsessive thinking, it's a part of OCD, I know because I have it, and have suffered at the hands of someone just like you.
Someone who could not let it go, and would not let me go.
It was hell.

Teenage hormones...when I was 8 a 12 year old boy whipped out his 'rock on' when I was kneeling down to tie my shoe lace.
He also pinned me down and rubbed it against me during a game of 'kiss chase'. And we obviously were not dating or anything.
This is just one example of a multitude of experiences I had throughout my teenage years.
Hormones make some people have very poor judgment and do stupid things. Sometimes they can even land you in jail.

Your girlfriend told him to put it away and he did. If they were close it's easy to see shear have forgiven his indiscretion. And fact is a lot of girls, and I mean ALOT would have caved to the pressure in that situation but she didn't.
As far as I am concerned this shows an extreme strength of character that deserves to be praised not punished. Very few teenagers make logical decisions in emotional situations, they aren't meant too.It's your learning curve in life, your meant to make mistakes.

Your girlfriend has been honest and straight up with you. Perhaps too honest. Some things are best left behind, and contrary to common beliefs you don't actually have to know everything about a partners past.

You need to stop and take a good long hard look at the situation. Make a list of the pro's and con's, although I suspect your list of cons will be short, they may well weigh too heavily and swing the balance.
You need help with your thinking patterns and coping mechanisms, you know why your like this so now you either take a break and deal with it so you can come back stronger.
Or, as I said at the beginning, cut her loose. Deal with this thing and move on.

I know it's easy to say, but you can not continue to punish someone for something they didn't even do, hell even if she had done something it would be wrong to keep dragging it up.

This is about you, your insecurities, your fear of other men and their/your sexual nature. Please please don't let this thing put guilt onto your gf that shouldn't be there.

I really do wish you all the best, I hope your able to do the right thing, and work things out, whatever that entails.