A good dissection of marriage can be found right here by Dick Houck, of the Catholic Defense League (not to be confused, evidently, with the Catholic League):
Man and woman naturally complement each other physically, emotionally, psychologically and spiritually. They were made to give themselves completely and fully to each other, for the two to become one. Marriage is merely one man and one woman making a complete and total gift of self to the other, for the rest of their earthly lives, and this is consummated in the act of sexual intercourse, which reflects and echoes this two becoming one in a physical way.
The two literally become “one flesh” and this one flesh union is the sole means by which human life is transmitted. This procreative capacity of marriage is the reason the government recognizes and bestows financial and legal benefits on it that are not bestowed on any other union. Those benefits are given to bind the husband and wife together, to bind the children to their mother and father, and to help the mother and father financially support their children, because children do best when they grow up in a stable home with their mother and father.Recognizing marriage and endowing it with government benefits helps guarantee such an environment for children, and government has a compelling reason to do this in virtue of its responsibility to safeguard and promote the common good.
Government is justified in denying same-sex relationships as equivalent with marriage for the simple fact that same-sex sexual acts, such as sodomy and mutual masturbation, are inherently sterile and can never under any circumstances result in the transmission of human life.
In the words of a venerable law professor from Minnesota “The government is not in the love business, but the baby business.”