There are so many reasons why, for a couple who wants children, a legal marriage is not in their best interest.
I know I mention this a lot, but I'm from a military town, and in this town live many young widows, who lost their husbands at 23 and 24. Sometimes these widows are entitled to healthcare for life, as well as one-time and lifetime payouts that enable many to live very comfortably without working. The catch? They cannot remarry, lest they lose the Widow's Benefits. Should these 24 year olds basically "close up shop," wearing black and always mourning, when they still have 50+ years of life ahead of them? Should they give up lifelong financial security in order to get married? Why can't these people be in a stable relationship and still have children? I am a proud product of a 2 parent family, but I believe a functional family can exist with two parents who are not necessarily LEGALLY married. The same goes for same-sex couples, and people for myriad reasons will not or cannot get married.
Marriage, as much of an industry as it has become in this country, is not the end-all and be-all of adult relationships.
I think being part of a stable relationship is important, as well as an extended family or friends to create a support system. Financial stability is important.
I believe (waiting for a flame here!) that either a COMPLETE COLLEGE EDUCATION OR VOCATIONAL EDUCATION is paramount. Yeah, we all hear the stories of high-school dropouts who are able to provide for a family, but they are the exception and not the rule.