First and foremost is because it violates the revealed Will of God as seen in Scripture. There are few things as clear in God's revelation as the sacred institution of marriage between a man and a woman. From Genesis 1 (the marriage of Adam and Eve) to Revelation 21 (the marriage of Christ the Lamb with His Bride the Church) God has had only one model of marriage, and it is not gay marriage. Even polygamy and divorce was done away with in the Christian Church, as Our Lord reminded all of the essence of marriage as it was "in the beginning." This Christian view of marriage became the norm of Western society and its most basic societal unit, undoubtedly allowing the West to flourish.
A close second to this is that gay marriage distorts the very concept of parenting as well as marriage. Even if you overlook the fact that gay marriage is by definition sterile, every child of a gay marriage has to be adopted or artificially inseminated, and that in itself is a violation of the whole concept of family. In the 80s, the Vatican actually said that the adoption of children by homosexuals "does violence" to the child. Pretty strong language, but true nonetheless. In its simplest terms, kids need a mom and a dad, and are forever stigmatized by being the child of a gay marriage. Kids model their lives, their concept of family, their morals and oftentimes their whole worldview on their parents' attitudes and values. These kids get a totally distorted view of all these basics.
Third but by no means the last reason, all of society suffers because of the public endorsement of an intrinsically disordered lifestyle and practice. Yes, "disordered" is what our Church calls both the homosexual orientation and lifestyle, and gay marriage simply ratifies that disorder on a social level. In recent debates, the term "human right" to describe gay marriage and the charge that anyone who stands against it is guilty of "discrimination" are examples of these fundamental distortions. Fr. Marx was right: a society cannot long survive this kind of violence done to its basic values, and history surely shows many societies like Ancient Greece, whose rapid decline was preceded by the proliferation of the gay lifestyle and its public acceptance.
While we respect all people as they are, we don't have to respect such a wholesale assault on everything that is sacred to us and good for our society; no, in fact, we must fight against it with our very lives. The question is not whether or not we can win the battle; the question is whether we will join it. Will we accept the challenge to defend the sacred or not? We are called to be faithful and obedient to the Plan of God for our world, and within that, God will bring forth the victory. There is no doubt that, if it is not already there, gay marriage will be coming to your state soon. If we don't fight it, our souls, our families and basically, our very civilization, will find themselves at "the end" of the line in very short order.
Rev. Thomas J. Euteneuer