The random working class voters Republicans add to their base the less socially conservative these samples are going to become on issues like this. Not exactly everyone wants a family now and those who do don't all believe that a family has to look like "Leave It To Beaver".
Marriage isn't for everyone. Some couples are better off being together and not getting married. For some people marriage changes the feel of the relationship.
If you aren't prepared for the responsibility of at least taking on a common-law marriage, how could you be prepared for the responsibility of bearing children?
That implies that you have to be married on paper to responsibly have children and that is not true. There are plenty of married people who are irresponsible parents or don't have the means to raise children. Marriage has nothing to do with properly raising children.