I realised I was poly when I fell in love with another woman. My wife and I were not good at communicating at that time so, to my regret, I failed to talk to her before beginning an affair. Of course she found out and it did make us reconsider our relationship and begin talking properly. Even after my wife knew about my affair I found myself effectively lying (or at least not being completely open) about silly things like meeting my other for coffee. It just seemed to cause more trouble to be honest than it was worth.
Sadly my wife was unable to deal with me loving two women and I have split with my second love because I could not handle the pain I was causing. I have issues with my wife now because she would not try to learn anything about polyamory. I hope I can overcome this resentment.
Believe me that it is possible to love two people, and for me it was the difference between the women that was so much part of it. So it was nothing that my wife was doing wrong, nor anything she was being that was the problem. I would not have wanted her to become more like my new love.
Anyway, just some thoughts from the other side.