I don't mean that it is important to convert lesbian and gay friends to a traditional understanding of sexual mores. Now, we should all want people to believe true things, and we should all believe things because we think they're true -- and what that means in practice is that, within the bounds of Christian love, good manners, and common sense, trying to persuade people of the Catholic view is appropriate; and it is equally appropriate that people who disbelieve the Catholic view would try to persuade people of their view. I don't mean that the truth is relative; but our responsibility is to live in accord with the truth as well as we can, having found that truth to the best of our ability. So if someone is honestly convinced of something, it is to be expected that they would live in accord with that belief -- we shouldn't be shocked or scandalized by that.
Charles Williams hinted in passing at the difficulty we are likely to experience in part of his commentary on Dante. In one of the more famous passages of the Inferno, when he first enters the circle in which heretics are housed, Dante encounters the soul of a man who was (on Dante's premises) both a religious and a political heretic. Williams has, among other things, this to say:
"The chief sinner to whom Dante here speaks is Farinata, a Florentine, an Epicurean, and an enemy of Dante's party in Florence. With our modern views of party politics, at worst, or with our English views of party politics, at best, it is a little difficult for us to remember that Dante thought his own political opponents metaphysically and morally wrong. He was also so touched by the habits of the Middle Ages (which he, of course, did not think were the Middle Ages; he thought he was a modern) that he believed it to be less important that men should think for themselves than that they should think rightly. We later moderns, on the whole, believe that men had better think for themselves even if they think wrongly. There is much to be said on both sides; this is not the place to argue it." -- The Figure of Beatrice, p. 126.
This catch, between the importance of intellectual honesty and the inevitable metaphysical consequences of ideas, is a difficult one. It does make the persecutions and Crusades of the Medieval era far more understandable, to consider that they at their best thought truth of such paramount importance that even honesty might conceivably be sacrificed to it; as it makes our own age more comprehensible (and, to my mind, helps correct the grossly exaggerated impression that our age is relativistic about morals to the nth degree), to see that we at our best consider honesty of such paramount importance that even truth may conceivably be sacrificed to it.
Here -- a little uncharacteristically, perhaps, but it is one of the ways in which I am a true child of my age -- I actually think the modern view preferable to the Medieval. I don't believe we really have to choose between honesty and accuracy. The truth, as Fox Mulder and St. Thomas believed, is out there. Which means that it is in principle discoverable by the human mind, according to its lights. "Christianity," said T. Kallistos Ware, an Anglican convert to Orthodoxy, "if true, has nothing to fear from honest inquiry" -- a sentiment we might do well to emblazon on our brains. Similarly, the Church at the Second Vatican Council declared, in Dignitatis Humanae, that "the truth cannot impose itself except by virtue of its own truth, as it makes its entry into the mind at once quietly and with power." But if we sacrifice honesty, we sacrifice our very capacity to receive truth.
And what does this have to do with gay marriage? Well, if two lesbians or two gay men have gotten married, it seems a safe assumption that they have no moral problems with doing so. And their consciences must be respected. This doesn't mean that we cannot disagree. What it does mean is that we should not be shocked, or contemptuous, or assume that they are being willful or irrational, because disagreement is not grounds to slander someone's character -- not even in the privacy of our own minds and hearts. It also means (though we ought to keep this in mind anyway) that we have no business commenting on their eternal destiny: we can read neither the future nor men's hearts, and the judgment of our fellow man has, rather emphatically, not been committed to us.
What about warning people of the consequences of their actions? Well, for starters, that sort of thing is not very effective outside the context of a relationship of mutual trust and respect, so I'd say work on that trust and respect first anyway (two-way street, remember). But more importantly, being a freelance catechist is not what witnessing to truth requires. If someone asks you what your beliefs are, then of course you should state them, without rancor and without apology, since truth requires neither. Equally, you should expect and accept that those who categorically disagree with you will likely state their beliefs without rancor and without apology -- it is not a mark of brazenness in them; it is a mark of authenticity of belief. And if someone doesn't ask, why are you bringing it up? Not every Christian is called to be an evangelist in the sense that St. Paul, or St. Patrick, or St. Francis Xavier were. There is a place for taking the initiative, but only ever with humility, charity, and wisdom. And it must be noted that the exhortations of the New Testament are directed chiefly toward living in such a way as to provoke questions, rather than talking unpromptedly to strangers about the answers.
In short, as my mother likes to say, "You are not the Holy Spirit."