MisterLove3000, you appear to have a Hollywood tainted perspective of what swinging is really all about. Most swingers are regular people, with caring relationships. Not the seedy stereotype that Hollywood likes to portray.
First, honesty is a critical issue for most swingers. Your girlfriend has been honest, good for her. However, your reaction may well impact how open she stays. If she thinks you can't handle it, she may just bury it. It is best, actually a must, for relationships to be open and honest. Try not to judge her, listen to what she says and share your feelings calmly and you will be well on your way to they type relationship most people aspire to.
Second, monogamy comes in the sexual kind, and the emotional kind. While swingers have non-monogamous sexual relationships they are almost always monogamous emotionally. That may sound strange and even unbelievable to you, but it is true. Most of us were sexually monogamous for years before getting into the lifestyle and most of us could return there if we so choose. Your girl friend has stated she wants monogamy, and the fact she was a swinger for a very short period of time is, in and of itself, no reason to disbelieve her.
Being a swinger does not turn a person into some kind of sex monster that can't have what most of world calls a "normal" sex life. She just experienced a few things than most will not. As for being satisfied with one partner, being a swinger does not mean one partner cannot satisfy their sexual needs. People swing for many reasons, but I assure you our partners absolutely can and do satisfy our desire and vice versa.
A few months of swinging is little more than an experiment, don't let wreck a good thing. But even if she had been a swinger for 5 years, it really doesn't matter. She was playing the field as a single person, she now has found the one person she wants to settle down with, be grateful it is you and move on.