I am not sure if it was a "fair" trade but it was a very badly designed one! :)
Surely you had better reasons for not wanting to get married than "My parents didn't have threesomes but if they did I would have seen their divorces in so much better light!" Either re-examine why you don't want to get married or get your head examined for thinking a threesome would change marriage.
In any case, if you want threesomes and other girls and have one you will want them MORE, and be even less happy not having more, and even less happy being in a marriage and not having any chance whatsoever.
So drop the crazy trade ideas, that will only make everyone maximally unhappy.
You need to independently decide if you need more girl variety in your life. I am unusual, but I simply never agreed to be monogamous with my girlfriend as I knew that would make me unhappy. There is no logic to that, but its true.
I also told her I never wanted to marry, for what I think are good reasons. I only want to be with her because I love her. A financially devastating divorce and a financially devastating business breakup have left me with no patience for locking myself into anything. I want to be with her forever, but no way am I signing a contract. And it seems crazy to me that love has to come with contract for anyone.
We have been together now for over 10 years and are very happy. At first she did threesomes and even a foursome (me and three girls!) but like many women got over the excitement of seeing me excited and doesn't want to do that anymore. My view was, I am who I am, but she is who she is and trying to convince her to do things she doesn't want to do isn't right so I don't.
I have a fling every now and then and she knows. She doesn't like it but we are open about it, all our friends know. We joke about it sometimes and to our friends, who are partly aghast at the arrangement and partly respect us a lot for being unconventional. (We live in a very conservative area.) And they also know how deeply in love we still are.
So my first advice is, be really honest with yourself about what you need as apposed to what you just want. If you absolutely have to have something to be happy then you need it. If you just would like it, but can get by, that is not a need.
Specifically, do you need to have other girls to be happy in life? Do you have to stay with your girlfriend to be happy if life? If there is a conflict there, can you be happy having one of those? What do you really think about marriage, since your trade idea really casts doubt on your serious issues with it.
Then talk to your girlfriend about everything. Be completely honest about how you feel, don't back down on your view on anything that really matters to you, while always respecting hers. That builds trust even when people are very different about something sensitive.
Keep a sense of humor about the impossibility of two people ever being perfectly easy for each other. Being able to laugh at "impossible" relationship issues really helps us. The funny stories we have are pretty damn twisted but keep a seemingly unbridgeable difference between us interesting instead of something that sours our lives.
I don't know that any of this helps you, but you are not the only relationship with this issue.