While I don't disagree with the sentiment, and I certainly think it is entirely provable that most people feel that way now, it does make it harder. When in life are you going to be care free and surrounded by people of the opposite sex your same age? Many years ago, marrying right out of high school was common. More recently, marrying in/right out of college was common. Now, it's common to wait until you are independently established, and find people with common interests. But it is harder to work 9-5 and find those people. Bars are tricky, online dating is a minefield...You just don't have the same core group of eligible people that you did when you were in college.
I'm not saying that's a good reason to get married, but I think it has something to do both with why so many people do it, and why so many of those marriages end badly.