Russell Shaw says that the Church needs to educate Catholics on the meaning of sacramental marriage. He comments that President Obama is framing the question around the meaning of the word ‘marriage’ – and that word definitions can change over time. However:
…the debate about same-sex marriage is not an argument about words. It is a debate about the fundamental core meaning of marriage—a meaning that isn’t even touched, much less changed, by playing games with words.
If that is true of civil marriage, though (and it is), it’s infinitely more true in the case of sacramental marriage.
St. Paul in his letter to the Ephesians (5:21-33) likens the relationship of husband and wife in matrimony to the relationship of Christ to the Church. (“This is a great mystery, and I mean in reference to Christ and the church.”) For centuries, the Christian tradition has seen here the foundation of its belief in the sacramentality of Christian marriage. The Catechism of the Catholic Church, reflecting that, speaks of “an efficacious sign, the sacrament of the covenant of Christ and the Church” (CCC 1617).
From this perspective, the very notion of a same-sex union somehow being a “sacrament of the covenant of Christ and the Church” borders on blasphemy.
But for whatever reason or combination of reasons, the deeply moving reality of sacramental marriage isn’t getting through to large numbers of Catholics today.