In a classic agrarian economy, having a spouse and making children works out -- more children = more hands = more work done = more food or more product to market.
In a modern "pure" capitalist industrial/service economy, the economic model doesn't pay for the cost of the reproduction of the labour it depends on. So not a surprise when people do the same cost-benefit analysis for themselves and choose (or are forced) to have no children, and to not bother with marriage as a result.
The no-children "choice" often also ends up with no partner -- married or not. I personally knew several women in their late 30s and 40s who ended up leaving partners or leaving the dating scene because the men they were finding were not interested in or capable of being fathers. On an aggregate scale this adds up to a phenomenon.
Some advanced capitalist nations have gotten around this through high immigration rates. Others are attempting to get around it with debt to finance social programs or stimulus etc. In both cases it's really just "shooting the puck down the ice" so to speak. The next generation or the one after has to deal with the problem.
Hmm. I wonder if this flows from divorce in the previous generation. If you didn't have a father (or at least not one who was present), you may not have seen a model of how to be a father. When you grow up, you are less likely to see it as something desirable, and less likely to know how to do it.
To avoid misunderstandings and red herrings, let me make some clarifying comments. Yes, I know that a number of those who left would have been lousy fathers if they stayed - even abusive. And that makes for a lousy model. And yes, I know that many of those without fathers present find others who can model fatherhood to them, and even of those who don't, some of them figure it out on their own. I still think that, on an aggregate scale, the absence of fathers a generation ago at least contributes to this trend today.