I believe that marriage is necessary for living together. Despite the popularity of couples living together without the benefit of marriage, our laws still support marriage. Marriage is necessary because it involves a commitment. Two people have made a vow to be together through marriage. Marriage helps create stable homes for children because you don't have a merry go round of partners all the time.
Marriage is a lovely ideal but it is more about legalities than about commitment. There are times when the legalities are important, as when there will be children. However, after one's children are grown or if there are to be none from this union, then living together will enable the couple to make their own commitment to each other.