Lots of cyber-ink has been expended in discussing the passing of the same-sex "marriage" law in New York and discussions, especially among Catholics, has been heated. The issue, it seems to me, is that there is confusion between the American cultural understanding of marriage and the Church's. The difference is that for most, marriage is only about human love, not a divine institution that is an integral part of God's plan for human beings.
The American cultural understanding is that marriage is primarily about ratifying the love of two persons. The Church's view, in contrast, is that while that is certainly true, marriage, as a sacrament, has a larger purpose. Therein lies the the difference. The understanding that many people who support gay "marriage" are working from is simply reductive.
Elements of the larger sacramental purpose include the formation of a family unit which will be fruitful and open to creating and nurturing new life (children), which forms the basic building block of society and images the love of God. Sacramental marriage is the natural result of the complementarity of male and female, who find their fulfillment in each other, and is the result of Natural Law and God's intention. It is not only the way we create loving environments for raising children, it is simply and most naturally the best way we provide those children with a model that validates and balances their own gender identity so that they can grow up to take their own natural roles in their own families. Along the way, sacramental marriage is a school for love and gratitude and a mutual journey of human and spiritual growth. The giving of oneself in sexual union is not only human, physical gratification, but a spiritual experience that mirrors divine love. You can read more about Catholic teaching on marriage here.
This is not to say that a committed same-sex couple cannot engage in at least some of the elements of a good marriage. This is not to say that their love is not real, and certainly not to say that they should not have the same rights in the civil realm. This is not even to say that they cannot raise children in a loving home (although in my experience, those children can sometimes be confused or troubled.) However, what those who deny the validity of Catholic teaching fail to see is that the purpose of a same-sex union is imperfect because it is primarily about validating love, and that the other elements are not present. The difference, of course, is mostly on two counts - the complementarity provided by two genders is absent, and the fruitfulness is, of course, missing. What is less obvious (and frankly a lower priority for those who see marriage as a purely human union) is that such unions are not the result and expression of Natural Law and God's plan.
The issue, as I and some other commentators see it, is that since many young men and women today are not choosing sacramental marriage in the Church, and since so many Catholics see same-sex unions as logical and necessary from a human standpoint, we have failed to pass on a sense that the Church's view of marriage is either valid or important. It is a fact that a significant number of marriages fail, mostly because the commitment is to nothing larger than love and sexual gratification. It is a fact that fewer Catholic young people are seeking sacramental (or for that matter even civil) marriage. That being acknowledged, it is important that we revisit the purpose of a lifelong union between man and woman.
This is, quite simply, a call for better catechesis of our own people about the meaning and purpose of the Sacrament of Marriage. We certainly cannot and should not tell non-Catholics what to believe, or how to conduct their lives in the civil sphere. What we can and must do is help our own people understand the special differences between a union that merely images human love, and one that images the fulfillment of divine love. Quite simply, the same-sex union of two people, though it may be loving and positive, is not the same as marriage.
A sacramental concept of marriage has something greater and more sacred to offer, and we need to preserve that treasure and pass it on to our younger generation. This is quite simply a call to textbook publishers, catechetical leaders, preachers, parents and others who help form young people to do better. If we don't, Catholic marriage will increasingly be seen as no different than anyone else's civil union.