Swinging is not for everyone, and it sounds like it is clearly something you can not wrap your brain around. That is not a knock, it is different from what most people are raised with. People make moral judgments all the time, and that is up to each of us as individuals. But, you seem to have bought into the notion that if people have different, more liberal, sexual desires than yourself then something must be wrong them, there must be a root cause for their deviant behavior. That can sometimes be the case, but not always. Or is it that you are making a moral judgment of her and trying to find a way to reconcile her behavior as a psychological problem in order to feel better about it.
Take yourself for example, you are 30, unmarried and have had approximately 20 sexual partners. For those that believe in abstinence until marriage and sexual monogamy, and a single life partner, your actions would be deviant or immoral. Do you feel like a deviant or immoral? Do you have low self esteem? I am guessing your answer is no. Would someone with a far more conservative view of sex think you are deviant and have low self esteem? Possibly, maybe even probably.
Why did you have sex with those other women? I assume because you enjoy sex, please correct me if I am wrong. So why is it so difficult to allow for the fact that your GF may have just been trying something she thought she might enjoy?
People tend to take their boundaries and call that the "norm." Your boundaries are far broader than some, and far narrower than some. What if you met a woman that you fell head over heels for but she just could not reconcile the fact that she was a virgin and you have a score of sexual partners. She might have issues with it based on her morals. She might even try to assuage those doubts by acribing your behavior as some psychological problem.
If your GF was going out getting plastered and finding herself in situations she later regretted, then you may be correct and she should seek some counseling. However, if she went into it with an open mind, understood what she was doing and wanted to experience it, then she is not unlike you, having sex for the simple pleasure of having sex.
I have read a number of psychologist that seem unable to grasp that people have what they term as "normal" sexual relationship for many reasons other than psychological problems. Don't fall into that trap. Talk to your GF and find out what makes her tick.
The questions you are asking are all about sex. But the real question is if you can trust her. The same thing every person has to learn about potential mates. I think she did an admirable job of opening up to you, she could have easily left her past buried, but chose to put her trust in you that you would understand and not over react. That shows a level of communication that some people take years to develop, I know because I am one of them. Be happy that you have found a woman that is open to talk to you about anything and trust you enough to tell he deepest secrets. You are a lucky guy.