There is a lot of confusion on the blogosphere about Catholic teaching on marriage.
Marriage is a loving union between and man and a woman. Period.
St. Thomas Aquinas is clear that Mary and Joseph had a real marriage despite not having children or not consummating the marriage. See my previous blogs on Josephite Marriages completely accepted by the Church.
The Maritains had such a marriage.
Children are the goal, but if God does not bless a couple with children, the marriage is still valid.
Do not be confused. We cannot argue against ssm from the role of parents, but from the sacrament of marriage. The sacrament of marriage is between two people with the blessings and openness to children. But, as I know many married couples who cannot have children and have adopted, one can see the grace of the sacrament in infertility.
The Catholic Church has organized adoption agencies for centuries, and supports adoption.
And there are several saints who were couples who chose Josephite Marriages.
Check Casti Connubii and the CCC. And my previous blogs on this....