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.
I agree with you on this, completely. I have put our relationship first and it's because of this that I have been able to manage through situations where many/most people would have given up.
However, I do feel like there could be a point where we are just too different that the relationship wouldn't work any more. I hope it never gets to that point, but I have come to realize over time that it is a possibility that I must prepare for, while at the same time doing everything in my power to make it work.