I see marriage as not really a promise to be passionately in love the rest of your life, but rather to be committed to the other person. A dedicated companion and partner for the rest of your life. It doesn't mean that you will never be attracted to or feel for anyone else. It doesn't mean that you will always feel exactly the same way you did at the beginning. People change over time. It does mean that you will put the relationship first, though.
With my situation, I'm not so much hurt that he found someone else attractive. I'm hurt that he gave up on me, because there are a hundred times I could have given up on him and didn't. I think being with someone for decades requires an unusual amount of dedication above anything else.