As for the original subject (No! I'm not a grammar nazi! <_<) I don't understand why people don't wait, except in the cases where the couple doesn't believe in marriage. There are people who don't want or feel they need to be married: they are devoted to each other and they don't care about the government's final say.
Personally, if you're a person who believes in marriage, you should wait until you're married to have a child. That way it grows up in a (hopefully) stable home with two parents.
Two parents is the ideal, though my brain just thought about the possibilities of a culture where the child is traditionally only brought up by one parent (no culture in particular, kind of a fictional one). If it were socially accepted then it wouldn't be a problem because that's just how it is for them.
Personally, I would have a problem with raising a child out of wedlock. A child is something I want to share with a partner, not something I want to do alone. *shrug*