In 2004, the Bishops of Pennsylvania issued a statement: Human Sexuality, Marriage and Same Sex Unions: Questions and Answers.  The Bishops want people to know more about “the Catholic vision of love and sexuality; the uninterrupted and universal understanding of marriage in human history; some of the challenges to marriage today, and why the Church is obliged to proclaim the truth about human sexuality, marriage and the good order of society.”

For example, the statement asks

What is the problem with calling a same-sex union a marriage?

Then answers

A same-sex union cannot be the uniquely complementary, mutually loving, and procreative relationship that God intends marriage to be as reflected in the way he created human nature. By definition a marriage is something other than a same-sex union. Because persons of the same sex cannot enter into a true conjugal union with each other, it would be wrong to act as if their relationship were a marriage. Since the coming together in sexual activity of people of the same sex is essentially different from the sexual activity of a man and a woman, simply saying they are the same does not make them the same. When society tries to redefine marriage so as to make other relationships equivalent to it, marriage itself is devalued.

The entire statement is available online.