Before I begin, I must leave a short disclaimer. As I am not a moral theologian, I cannot say for sure that my musings on this subject are correct, at least from a Church perspective. I, however, will strive to do my best to represent the teachings and directives of the Church.
The subject of homosexual marriage or same-sex union has been in the news a lot recently, especially with all the ballot measures that passed in various states. I have been reflecting on our role as the Church Militant to defend the Church's teaching in this matter. It tends to be a very personal issue with many people. I think this is because of a misplaced compassion and the fact that so many people either know or are related to persons with a homosexual attraction. This makes it a very divisive issue, and one in which people will attack you for speaking out, just because you spoke out, or at least this is my experience.
There are a few questions me must address in order to understand our role as the laity. First, we must ask what the Church teaches regarding persons with same-sex attraction. Next, we can consider the question of secular rights of gay persons. Finally, we will try to formulate the Catholic argument regarding the answers to the above questions.
What does Holy Mother Church, in her wisdom, give to us as guidance regarding homosexuality? The Catechism of the Catholic Church (CCC) discusses homosexuality directly in paragraphs 2357-2359.
(2357) Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity, tradition has always declared that "homosexual acts are intrinsically disordered." They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.
(2358) The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God's will in their lives and, if they are Christians, to unite to the sacrifice of the Lord's Cross the difficulties they may encounter from their condition.
(2359) Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.
These shall form the backbone of our understanding of the Church's teaching in this matter.
Homosexual acts are gravely depraved. This means that they are the grave matter required for Mortal Sin. Under no circumstances can they be approved. This must guide our thoughts. To say "as long as you are in a committed, permanent, etc. relationship..." would be wrongheaded. Under no circumstances.
The argument comes from the Natural Law, which is knowable through reason (c.f. CCC 1956). The creation of Man and Woman, their nature and purpose, is written into their bodies. It is clear from the use of reason, that the reasonable (in the sense of using reason) use of the human body is the conjugal union of man and woman. The Servant of God, Pope John Paul II, of blessed memory, wrote in his catechesis on the Theology of the Body about the sexual complementarity of Man and Woman. This complementarity is not existent in a homosexual union. Any reasonable person, considering the rational arguments, should come to the conclusion that Man and Woman are made for each other.
The homosexual inclination is objectively disordered. This is a philosophical term, and probably should be avoided in an argument, simply because the word "disordered" has many negative connotations which can bog down a conversation. What is meant here, by disorder, is the literal meaning dis-, being against, order. The order of the world is not toward homosexual unions, but toward the union of a man and a woman. This is the natural law argument put forward above. All desires toward sinfulness are disordered. The baptized must continue to struggle against concupiscence of the flesh and disordered desires (CCC 2520). I cannot say that the desire felt by homosexual persons is the same as any other disordered desire, but we are all called to fight our disordered desires, of whatever sort they are.
Every sign of unjust discrimination in their regard should be avoided. Is it just to allow someone to continue in sin, or to even encourage this sin? No. This is why the Church opposes state supported institutions which encourage or promote a homosexual lifestyle.
The catechism points 2358-9 can really apply to anyone. We are all called to chastity. We are all called to self-mastery. We are all called to approach Christian perfection.
Finally I must make a point regarding Church teaching. The Church tells us that Marriage is a sacrament, and as such can only be validly received as a couple which is one man and one woman. This is not a question of licit/illicit, but rather of valid/invalid. Just as no woman can receive priestly ordination, no homosexual couple can receive marriage. This is what the Church teaches, and as such, no matter what the State says, no homosexual couple can be married.
This still leaves the question of secular rights that should be granted to persons with homosexual attraction. We consider here the things that gay-rights activists claim are important to the requiring of gay marriage. We must remember here that the catechism does distinguish between homosexuals who are Christian and who are not. We will assume we are talking about the case of non-Christians at this point.
Homosexual persons want the right of inheritance, that is, they want their partners to be able to inherit their things, and be protected as such under the law. My first question is this: are people prohibited under the law, today, from leaving their inheritance to whomever they wish? Is it not true that anyone could be named a benefactor for life insurance. If a homosexual person desired someone specific to be a medical proxy, would this be allowed?
I could be wrong, but I think the answer to all of these is yes. What is it, then, that people want? Married filing jointly seems like a feeble reason to get married, though the ethics of our tax code is outside the scope of this discussion. Perhaps extending health care in a job to same-sex partners, though marriage, is what is desired. This is perhaps a noble pursuit, and is one of the only practical reasons I can see that people would want homosexual unions.
I haven't addressed the most important reason the homosexual lobby wants gay marriage. They want to have homosexuals and homosexualism to be seen as "normal" "mainstream" or "acceptable". They are smart in that sense. Recall what happened around when Roe v. Wade was decided. It was a sad, unacceptable procedure before then, but as time went on, its legality caused its use which led to its becoming accepted by society. Now, people see it as an option. Now you have people who can be "personally opposed" but are in essence accepting of the practice. The same is desired of homosexual marriage. We are not at this point, and California's Prop. 8 is a good example of this. In places where laws were snuck in (through the courts for instance) the people saw to it to overturn them. Even Californians can see that homosexual acts are otherwise unacceptable and shouldn't be legally recognized or legitimized.
We are left, finally, with addressing the role of the laity in regards to recognition of homosexual marriage, etc. I have heard on the radio of a Catholic mutual fund, which has as one of its criterion that the company not support benefits for non-married couples (gay or otherwise). Yet, who can oppose the concept of extending health care benefits to more people (whoever they be)? We must always go back to the line in the catechism: "Under no circumstances can they be approved."
What does this mean to us? It means, indeed, that we can't, as laity, just say "I don't care what you do in your bedroom." Under no circumstances. Indeed, when confronted with such situations, we must oppose them. We cannot support political action leading toward a legitimizing or regularizing of homosexual relations. We should not, in the course of our conversations give assent to homosexual relations. This is what we are called to witness to.