I'm not going to argue the Catholic Church's teaching on marriage. In my opinion, you either accept it or not. If you don't accept it, my only hope is that you at least have tried to understand it and prayed about it. Marriage is a big deal, and requires much more discernment than a lot of people give it. In the interest of full disclosure, I have a number of friends who happen to be homosexual- I have no interest in making them any less happy with what I'm writing here. (If you're ultra-conservative and that last line bothers you, try to keep reading. I probably won't be scoring any additional points with people who are gay/lesbian.)
I do want call attention to some of the myths or distortions of some of the arguments for same-sex marriage that just don't hold any water for me from a Christian perspective.
Marriage equals happiness. Actually it doesn't, at least not the way temptation makes it out to be. If it did, then the divorce rate wouldn't be so high. If marriage is your vocation then it's your path to holiness. There are plenty of happy times which are previews of the eternal happiness to come, but it's far from a fairytale. Marriage is a road with some parts of it pretty rough. Some might disagree, but I think it's harder being married than it is being a celibate priest or vowed religious.
Marriage equals equality. I'm not seeing this either. Talk to someone who has experienced racism or bigotry. People in other minority groups have been allowed to marry; it hasn't made them seem any more equal. People who are already married aren't any more equal than people who aren't. What makes us equal is the fact that each of us is made in the image and likeness of God, and that each person is unique and unrepeatable.
People should be able to get married if they want to. That's actually not true. You can't get married if you're currently married to someone else. You can't get married if the person you want to marry doesn't want to marry you. You can't get married if you are below the legal age to do so. You can't get married to someone in your immediate family. There are many other situations, but these go to show that just because you want to get married doesn't necessarily mean you actually should.
People should be free to do what they want. Let's be clear about freedom. Being able to do what you want and when you want is licensure, not freedom. Real freedom is being able to be who God calls you to be, without the interference of sin or temptation. Jesus Christ conquered death not for us to have licensure and do whatever we want, but so that we have an opportunity to have real freedom (a.k.a. eternal life with Him).
A genuine marriage is a lot less about doing what you want, and more about self-sacrifice and giving of oneself for your spouse and the family. If that isn't going to be the case within same-sex marriage, then we'll be seeing a lot of same-sex divorce, too.
