Sure! But, since you seem to want a lot of info, this will be the long version
I was a budding alcoholic for a long time, but it didn't get to be serious until after I met my wife. I was working construction, and I hurt my back badly, at a time when I didn't have insurance. Since I couldn't afford an MRI and the PT or surgery that would then follow, I chose to self-medicate my pain with alcohol, in large amounts.
I dealt with chronic back pain for 4+ years, getting deeper into alcoholism the whole way. At the time when we first discussed poly, I had been pretty much a leech on the life of my wife and her kids for at least a year. I spent most of my time either drunk or hungover, and neither one of those states is conducive to meeting responsibilities- which I didn't. Meanwhile my wife worked 15-hour days, usually coming home to a house that was worse than how she left it, a drunk husband, and unhappy kids.
I think it is understandable that she wasn't exactly happy with this situation. She had needs- such as for a person to be a partner with her, someone who would make her life and home better by being there. I was not that person. She also needed people who were kind and loving, and I was also not that person.
Despite all that, I still loved my wife dearly. With the idea of poly fresh in my mind, I decided that it made perfect sense that she should look for her happiness wherever she could find it. I knew then (and know now) that her capacity for love is such that she can love whoever she chooses, without detracting from her love for me. So I told her so.
I am by no means a perfectly non-codependent person now, but I have done a lot of work since then. I am a partner to her, and I (at least try) to make her life and home better by being in it. There aren't the same unmet needs. Now, the reasons for poly are different.