In replying to the government's consultation on marriage I wrote something like this and it most likely will be ignored:
As a Catholic, marriage has a wide theological significance. The marriage between a man and a woman can not be separated from an understanding of the marriage of Christ to His Church or an understanding of the marriage between Heaven and Earth. Marriage is a divinely instituted covenant between two parties that are essentially and substantially different, the creative loving action of God flows through marriage to produce a unity between the two distinct parties. Therefore it is impossible to accept a definition of marriage that allows for two creatures that are essentially and substantially similar to come together under that union.
I did not say that relationships between members of the same sex are not creative. They do not produce the ultimate fruits of creation; children. Nevertheless they can be life-affirming, loving, sacrificial and creative (and let's keep this celibate, I'm definitely not referring to sexual acts here). They are not however sacramental or unitive (two creatures essentially and substantially the same can not become more one) and therefore as a lasting bond between two individuals they are contrary to God's ideal.
Why am I trying so hard to define marriage without much reference to its primary function; offspring?
This is because, any secularist will point out to you perfectly functional nice couples who produce perfectly rounded, "nice" children "without the need for God". Biologically, children happen without any need for religious input from the human parties involved.
Christian marriage has to be about sanctification. For Catholics, the mutual sanctification of the spouses through the Sacraments adds a dimension to the bringing up of children that would not be there if this were not the case. As Catholics I feel that we must see way beyond the earthbound view of traditional marriage (man, woman, love, children...) and whilst marriage is only a bond whilst the two parties are alive on earth, it is nevertheless aligned to the supernatural in the most fundamental and beautiful of ways, as the unity therein gives glory to God.
Is marriage not ultimately the most profound expression of the "priesthood of all believers"?