The other side of the coin is that maybe we should acknowledge that marriage historically was about combining property and legitimizing children during that long stretch of history in which most people didn't see 40 years of age, and 65 was prodigiously old. 14 year old parents were common, because you got on breeding as soon as possible. Monogamy was a sanctioned expediency because no one lived long enough to "stray"...and getting fed trumped getting bored.
Maybe 50 years with the same person is what's cruel and unusual. Maybe we are hard-wired as animals to enjoy multiple partners and less-binding ties. If we agree that supporting our offspring is the critical bit, does it matter who we are sleeping with?
Robert Heinlein figured all this out some time ago with his concept of "Group Marriage"
http://en.wikipedia.org/wiki/Group_marriage , although he was by no means the first, but he provided fictional treatments showing how extended family units might work. More thoughtful than the "communes" of the '60s, his conceptions are worth examining, and are predicated not on biblical inheritance laws (and most of the biblical guys had as many wives as they could trade livestock for!) but on self-interest, a sense of belonging, and a rational approach, if that makes sense, to happiness. These days, many people would like to experience the attachments of marriage, but don't personally want children of their own...the legitimacy of which (tied into property and inheritance) was the main reason to get married until 1900 or so.
Maybe the reason so many marriages fail is not due to some pervading shallowness of modern, spoiled, short-attention-spanned individuals, but is a simple acknowledgement that marriage itself has some unworkable aspects to it for most people. I admire long-lived marriages, but I don't see them as the ideal, because I think that it is rare. I also think that some people stay married out of habit, not because they are happy or even in love.
Maybe we can take a lesson from the Celts, who found the yearly "handfasting" a reasonable approach: an annual contract to pledge troth, but it had to be renewed by both parties anew each year to remain in effect.