Anon -
When I was going through a rough time (I had just been diagnosed bipolar, was living in a supported accomodation hostel, multiple psych admittances, often taken into police custody for protection etc) I did similar to your girlfriend. I went out with friends, got drunk, went off with other boys, etc, all whilst having an amazing loving and caring boyfriend.
I was honest with him, and he was devastated. howver he did also accept that I was incredibly self destructive at the time, and yes there was truth in the fact that I was looking for approval, acceptance, and that these other boyswere my 'safety plan' in case my own boyf ever left me. At the time, I was also trying to get my boyf to break up with me, as I couldn't break up with him, because I couldn't face his heartbreak, so I did what I could to get him to break up with me.
Fast forward 2 years, and we are in such a happy, healthy, trusting and committed relationship. I worked through the issues I had at that time, and now I am working on my more 'core' beliefs, with the aid of MH services.
My boyf accepts it wasn't cheating, as I love him with all my heart, I care for him much more thn I do for myself, and when I was with the other men, I was just self-destructing, in the same way I am when I am cutting, or any of my other behaviours.
I think the key is to be open. This girl has a lot of problems, and probably has low self esteem, and difficulty accepting that anyone could love her. Her 'activity' with the other men shows that - there are complex reasons behind it, and there is no way she is *just* a slag. Is she in therapy or counselling? Could you go to it with her, for a couple of sessions?
Don't accept the views of people who have never been in the situation, who look at things with the eyes of ignorant, stigmatised idiots. If you think she is worth it, andif you genuinely believe that she wants your relationship to work - then work on it, it could be the best thing you have ever done.