Hm, I guess I know some of those 'life-long', happily and some decades together people. As some are still alive, the life-long may be in question but they are all 50 to 70+ and happily married to their first wife/husband. That's why I have had such a hard time coming to terms with my desires for a second man, while loving my husband. I was surrounded by all those picture perfect mono marriages. My grandparents were of the life-long kind, my grandmother died shortly after my grandfather because she was missing him so much. My neighbours (the oldest couple, age of my grandparents), whom we are all friends with, are inseparable and their daughter and her husband (a bit younger than my parents) reach 20-something this year in their marriage, my parents celebrated silver marriage last year and so on and so forth if you look around in my neighbourhood, relatives, friends – most are happily married. (Not this fassade like happy marriage, that I am sure of.)
That's why I found it so hard to not be the norm. When I realized that it didn't matter, that the love was still there for my husband, I felt kind of relieved. It was possible to be happy even though it wasn't fitting in the norm I was used to. From then onward, things got better for my emotional health. We opened the marriage, even though I have the feeling that we closed it again right away to a 'three-person-marriage' kind of, but we are happy.
I am with NovemberRain: People are endlessly fascinating. My husband isn't the man I married and isn't the boy I met when we were 17. We reach 12 years in September this year and there are many things that changed and that I came to love when I discovered them over time. (Some I don't like as well, but well, that's the nature of the game
). I always understood how lifelong attraction could be possible, but I guess it is some kind of a gamble. There is no guarantee that you will fall in love with the changed and new person your partner will become with time passing by, but there is the possibility that you will. It happened for us till now, I hope this will continue in the future as well.
And I think, it is kind of double fascinating to have the opportunity to experience this multiple times. I am still in the phase of learning about my new spouse. Things are fresh, but I experience with him, what I experience with Sward, but 100 times faster, as he has so many things I still need to discover and kind of 'rate' how much I like or dislike them. Sward and I know each other well, therefore new traits of our personality don't develop that fast for each other. But it is great to find new ones and expand the picture of the person you love a bit more and get the feeling to love him/her even more because of that. It makes me feel closer to him every time it happens.
So, to answer your question: It is possible, in the traditional way or the non-traditional poly one. At least from my point of view. As attraction (emotional, physical) comes by itself.