Christ chose to be born and grow up in the bosom of the holy family of Joseph and Mary. The Church is nothing other than "the family of God." From the beginning, the core of the Church was often constituted by those who had become believers "together with all [their] household." When they were converted, they desired that "their whole household" should also be saved. These families who became believers were islands of Christian life in an unbelieving world. CCC 1655
The Christian family is called the domestic church because the family manifests and lives out the communal and familial nature of the Church as the family of God. Each family member, in accord with their own role, exercises the baptismal priesthood and contributes toward making the family a community of grace and of prayer, a school of human and Christian virtue, and the place where the faith is first proclaimed to children. (#350)
Not all couples can have children, of course. It may be that God has chosen to bless them through birth much later and with greater difficulty than they wish, or to adopt a parentless child, or even to remain without children. Being open to life is what couples are called to in this, as hard as that can be at times. Another gift of marriage is the bond between couples, which helps them in this life and hopefully in achieving Paradise in the end.
The Catechism Companion, Vol II comments on this:
The Church supports the many spouses who are open to life but have not been able to have children biologically. Those couples can still embrace a meaningful marriage that is filled with love, selflessness, and care for others. (p. 216)