Marriage is important in building a good family. Marriage is not based on the papers that you and your wife signed on. It is not just a piece of paper that I have been hearing from separated couples, but this is more of an agreement between the two, the union of their hearts and a matrimonial ceremony based upon the will of God.
If the couple are executing premarital sex, then marriage should be there. if they do not want marriage, sex should not be there too.
Marriage is not just following the cultural norms, but it is under the law that protects your soon to be kids. It is important if you love your woman and wanting to put your relationship to the next level, with the proper grace from the parents, society and from the Lord most especially..