If they know who knocked them up, they can try to get paternity. If they had mutliple partners (the slut syndrome) a DNA test may be in order.
Besides married couples pay more in taxes than single people. We have a marriage tax, which makes some people decide not to get married. This way the tax codes are encouraging young women not to get married.
In a perfect world, mothers are married and have a husband, in the real world that is not always the case. Call it different morals, different ethics, what comes around goes around, etc.
When I got engaged to my wife, my grandmother asked if I was living with her. When I said "No", she said "Why not? Everyone else is doing it?" But I lived at my mother's house before I got married. Then we moved into a house my wife was renting, and then later bought a house with a morage in the end of 1998.