There are probably several factors to this that are often overlooked about marriage today.
People live a lot longer today than they have throughout the rest of history. When you got married in the Middle Ages, the odds that your marriage would last beyond a couple of decades were probably quite slim, since life expectancy was less than 40 (even when you remove the effect of high infant mortality) and only one of the spouses needed to die for the marriage to be over, and it would usually be before all the kids were grown and out of the house
Now many couples (if they stay together) will live well into their 80s, and those who marry early will have 50 - 60 years together, including 20 or more years when they are together, alone, after the kids are grown and gone. That is a relatively recent phenomenon in society.
Secondly, the vast majority of people today have more leisure time than any other generation in history. That means couples are spending a lot more time at home together in each others' company, and given the changes in schooling, and the proliferation of organized activities for kids, less time devoted solely to bringing up and looking after the kids (for the mother, at least).
While for some couples that extra time together is no doubt a blessing, for others -- those who may not grow together after that first bloom of love fades (which undeniably happens) -- it only adds to the pressure of the marriage. I know three couples who have been married several years, and are likely to stay together as they raise their kids, but once that shared responsibility is gone, and they have a lot more leisure time to spend in each others' company, I already suspect that maybe two out of the three will end up separating at some point.
Finally, of course, communities are very different today than they were in past generations. Mobility is far higher, as is the population density, which means that both spouses will meet and get to know a lot more people throughout their lives (though probably not as well) giving them much more opportunity for meeting someone else they may fall for. And the relative anonymity of living in a large community probably helps facilitate that.
So marriage is a very different proposition today than it has been in most of the rest of history, with the biggest change being the greatly increased amount of time couples will have together over a life long marriage. One can make all the theological arguments you want about how marriage has always been the same proposition (which, of course, in some ways it has) but the reality is that with the great increase in life-expectancy in the last 50-100 years, along with all the other changes in modern society, it is far harder to achieve a life-long marriage today than it ever has been in the past.
I am not surprised to find that more and more people are seeing marriage as being less important today. It's probably mostly a reaction to those changes. I suspect that the vast majority of those who feel that marriage is no longer as important still cherish those who do manage to make it work for a life time. My parents know a lot of divorced people (many of them neighbors, most of them women) and they have told me that they are often complimented by those people (many of whom have every reason to be cynical about marriage) for their (now) 29 years together.
Therefore, I think many people still believe that life-long marriage is an ideal worth striving for, but that they are being realistic about their chances of achieving it themselves.
