My husband and I started with monogamy, he was very jealous in the beginning of our relationship. We were very young (I was 16, he 20) and he had big insecurities. He was jealous of me spending time at school or with friends. There was some serious co-dependence in the beginning, I think that is why I put up with the jealousy. His self esteem gradually got better and he stopped the controlling behaviour within the first year, but it took us a few more years to repair the damage and for me to believe that he wouldn't start that kind of behaviour again.
We got married after we had been together for 4 years and started talking about the possibility of having a threesome with another girl at around the same time (I am bisexual). It was all very hypothetical at that point and first we only played with the idea. But I started to think about it more analytically and finally told him that the idea of him having sex with somebody else didn't actually bother me at all. I was sort of surprised myself, even though I had never been a jealous person. I gave him the "permission" to be with somebody if he felt like it. I didn't expect any freedom in return: my reason for giving him permission was that I didn't see a reason to limit him since the thought didn't bother me in any way. My husband started thinking about the whole thing, and after a while he told me that he wouldn't feel bad if I were to kiss or have sex with another girl, but felt jealous thinking about me with another guy. At that point neither of us had any knowlege of polyamory. I found about it later and shared with him that it would also be ok if he fell in love with somebody, but we didn't discuss it further.
We had the agreement for almost two years before either of us had sex. My husband hasn't still been with anybody else. I had sex with a friend, and that was a one-time thing. Then after more than a year, I met an online-friend of mine for the first time and there was sex as well but also deeper feelings. So after visiting her I talked to my husband about polyamory and discussed how he would feel about me having a serious relationship with her. My husband didn't feel any jealousy about the situation and was okay with it. My friend had similar talks with her husband, we spent some more time together and now she is officially my girlfriend.
I feel very happy, I like her a lot and feel that there could be so much potential in this. She is a wonderful person and seems compatible. Although that might partially be the NRE talking.
I also feel that taking this step we have gained such openness in my marriage. It's so wonderful to see new sides to a person you have been long with. And I feel amazement at how far we have come, and especially my husband. Both of us have obviously grown and changed a lot, but in terms of the jealousy he has made tremendous progress. I may have given him technically more freedom but he has taken more steps to be where he is now and I am very proud of him.
And I do feel that I am the luckiest person in the world.