One of the hardest things to wrap your head around is the idea that it is OK to romantically love two people simultaneously, and I can see how a mono partner would have the greatest trouble with this. I know it is a major issue for my wife at the moment as well.
I struggled with this while harmlessly flirting and not really looking for anything but NSA fun. But then you find someone who you really click with, and before you know it you are falling head over heels for them and acting like a 16-year old kid. I began to question my own sanity and wonder WTF was going on in my head. Was I falling out of love with my wife? But the more I thought about it, the more I realized that the emotions were calmer there, but I still felt a deep love for her. AND the new girlfriend. I love them both.... and that's OK. Society and a culture built upon the false foundation of "monogamy is the only way" pounds into us that this is not possible; which is why a society that frowns upon poly has no problem with serial monogamy (many of whom probably would have been much happier poly)...