Clownfish, I can relate to your dilemma.
I've experienced something similar and it is f*cked up.
To those saying harden up etc, I say if you haven't been in that position, maybe shut up with your judgements and try and understand WHY it is a problem for this bloke, not criticise him for being honest, for f*ck's sake get off your high horses.
As I said, I've experienced something similar, for me it was a few years into a rslationship and yes it can profoundly change how you see your partner. I am very, very far from conservative, but women who fall within my definition of 'too easy' are, to me, something I stay clear of. Ex-boyfriends who loved and cared for my girl didn't bother me, but I hate the idea of girls getting totally pissed (drunk) and getting banged by some guy looking for a breathing masturbation device.
I have a younger sister and anyone who did that to her would disappear.
When I found out that I'd been misled and lied to I was furious. We aren't talking about a small lie either although it wasn't quite as substantial as this example.
Question is, where does this leave you. For me, I still loved my partner, but this new info changed how I saw her, in a huge way it felt like I didn't know who she was anymore, which was a horrible feeling after I'd made some enormous sacrifices to be with her and after a not-insignificant amount of time.
I decided not to end it, and instead took the hard road and started to talk - over a number of months - about her experiences, how they were for her, what they meant to her, to understand her again. It wasn't pleasant to hear this stuff but it helped me understand who my parnter was again. I also went a saw a therapist to talk it out a bit, to help deal with the anger and hurt I felt at being lied to, and other unresolved stuff that came up (from a previous relationship). This was very helpful. She also started seeing a therapist because her experiences were not particularly pleasant and she wanted some resolution around them.
I'm still with my girl and love her more than I ever have. BUT, my concept of her now is very different to how it was before - not good or bad, just different.
It's all well and good to tell someone to forgive them, throw numbers at them about how many men she should have f*cked by now (which is bullshit IMO anyway), or tell them to harden the f*ck up... But it is POINTLESS. Mate, you have a potentially difficult task ahead of you, but it's not hopeless by any means. It is all about YOU. You need to find out why it is bothering YOU, and it's not wrong or right, it just is the way it is. Find a therapist or someone you can talk to, write down your thoughts and how you are feeling to get it out there. You could well find yourself in a happier place than you were before, it's ok to be angry and sad etc, just be kind to yourself and to her while you figure it out.
Good luck.