The whole point of them having children before marriage, is about power. When she does marry, the future husband begins his marriage in second place behind her child.
Secondly, the first child tends to be very abusive of any half-siblings that arrive later.
My dh works with a 25-year old, single mother of two. They both at least have the same father- an unemployed, felon loser who doesn't lift a finger to do anything, with the kids or otherwise. When my dh asks why she has chosen this, she says that she wants to have her kids while she's young and doesn't necessarily even want to marry this guy- she just feels like having kids now and doesn't want to wait. She does want to have at least one more baby with him before she's 30,- then she says she'll think of marriage (probably with someone else if he doesn't shape up- as if they'll be lining up around the block for her to choose from). She sees absolutely nothing wrong with this and thinks unmarried couples should have all the same rights and respect as married ones- because it's just a choice- not right or wrong. Oh, yeah, this pillar of motherhood also lets her 6/7 year old watch movies like Saw and Texas Chain Saw Massacre because he likes them.