It's a form of demographic debt. By increasing the workforce #, you decrease the amount of kids people have (because they can't afford the time to have many kids) but increase total GDP until you pay the piper later with decrease reproduction rates and more seniors that the economically productive.

I also want to say that it's not necessarily good to have one person be the house spouse only and then come out the other end with no skills. That can be a trap and be especially bad if a divorce or other split up happens, not to mention when your a bored empty nester.

It's good to be have some skilled profession to come back to. Kids are time consuming from the start, but they start spending less and less time with you as they get older.