Interesting discussion - everyone is raising excellent points.
SugarCane, I somewhat agree that with fewer people getting married, those that do may be marrying for the "right" reasons, though as Mark explained, being "in love" is not necessarly the right reason. The right reasons may have more to do with values and religious beliefs - which is not to say marriage should be a cold, arranged institution or anything but that it should have some tie-in with your personal beliefs. At least if it does, there is more glue to hold it together "until death do us part."
There are many reasons why people chose to live common law rather than marry - everything from pure laziness to personal opposition to the institution of marriage and all that falls between. Some live together just to save money and/or screw the government out of taxes which, as callous as it may sound still beats marrying someone for the same reasons.
I can't speak for anyone else, but I know my own view on marriage has been badly tainted by my experiences, and I'm not just talking about the affair because people who are not your spouses can still cheat on you. What grinds at me is that despite the necessity for both parties to agree to enter a marital contract, it only takes one to break it. There is no pentalty whatsoever for the one who does and the thoughts, feelings and opinion of the other are completely disregarded. There is nothing to protect a BS - not legally, emotionally or in any other aspect. The actually contract - the marriage certificate that you sign with witnesses etc., has absolutely no value whatsoever, save whatever sentiment it holds to you personally. With all that being the case, why would I enter into such a contract? That's like signing up for a credit card, but they can double charge me any time they want and I just have to pay. Or signing up for snow removal, paying for it and it never happens. It's a fraud, but it's a legal fraud.
In my ideal universe, marriage means what I thought it did when I first married; what it means to most people on this board - a lifetime commitment to your spouse - for better and for worse, and all the other vows. I so want to have that but it doesn't exist in the real world so it's better that I do without.
Maybe that's why I love this board because there are others here who hope for the same ideal universe.