One of the less effective arguments often goes something like this, and is conducted between a traditional marriage supporter (TSM) and a gay-rights advocate (GRA):
TSM: Two people of the same gender can't get married. That changes the definition of marriage.There are better ways to argue against gay marriage, of course. But I show this argument, which is a composite of many such discussions I've seen online and elsewhere, to show what happens when you insist on having a precise definition as the starting point for a discussion about a moral issue.
GRA: What is the definition of marriage?
TSM: Marriage is a legal and sexual union between one man and one woman.
GRA: And where does this definition come from?
TSM: Our culture and civilization. Marriage has meant this one thing for a couple thousand years, at least.
GRA: But if our culture wants to change the definition, then it can.
TSM: But having same-sex marriage changes the definition too much. If marriage isn't between a man and a woman, then what is it?
GRA: It's the legal and sexual union of two people. Gender isn't important.
TSM: But gender is important to the definition of marriage.
GRA: To a definition, maybe. But there have been many definitions of marriage throughout human history. And we no longer think gender matters in most areas. So why should it matter in marriage?
TSM: Because marriage is about procreation, about having children...
GRA: Now, wait a minute! You said the definition of marriage was the legal and sexual union of a man and a women. There's nothing about children in the definition. If there were, then infertile couples and elderly couples couldn't get married.
TSM: Just because I didn't mention it didn't mean that it's not part of our understanding of...
GRA: But it's not in the definition. By the definition, marriage is pretty vague. There's no reason for it to involve a man and a woman, and not two men or two women. There's really no reason for it to be limited to two people, except that our culture isn't ready to take that step.
TSM: You're altering the definition so much that you're making the word marriage mean nothing!
GRA: Well, you can't seem to define it in such a way that it means only what you want it to mean. So maybe it doesn't really mean anything...
The same thing happens in the torture debate, from the opposite side. The torture defender insists that without an extremely precise definition of exactly what torture is in each and every possible hypothetical situation, we can't possibly say that torture is wrong, that it is morally evil, gravely so. But just as the definition of marriage is hard to pin down in a single declarative sentence drafted in such a way that it reflects the religious, moral, philosophical and cultural understanding of the Christian West and not, say, the understanding of ancient or modern pagan and/or polygamous cultures, etc., so is it difficult to pin down a definition of torture that, while being specific and legal, reflects the moral vision of Christianity. This is especially true of the vision of the Catholic Church, which clearly wishes to promote the idea that we ought to treat all people, including prisoners, humanely, and that various acts of violence, pain, coercion etc. violate that principle.
To put this more simply, in the gay "marriage" debate proponents of same-sex "marriage" want to start with a definition in order to deconstruct that definition; that is, they want a definition of marriage so that they can reword, alter, and destroy the definition to the point where it is meaningless enough to include the understanding of "marriage" they wish to promote. Similarly, in the torture debates, many (though not all) who want a clear, precise, exhaustive, and definitive definition of exactly what torture is want this definition so they can insist that certain things, such as waterboarding, or putting prisoners in freezing cold cells and drenching them with water, or humiliating them in various inhumane ways are somehow not torture, because our definition didn't actually mention these particular things.
In the discussions concerning marriage, it is important to remember that marriage, an enduring cultural institution centered around the building up of strong natural families and the raising by a man and a woman of their own biological children whenever possible, can't be reduced to a mere definition of a sentence or so. It is equally important in the discussions concerning torture to recall that humanity's capacity for the intentional infliction of suffering on our fellow human beings can't be summed up by a quick definition of what torture is. In both discussions, there is a tendency by those on the other side of the issue to insist on a definition merely as a starting point for the deconstruction of that definition--and the point of that, of course, is so that the perversions they wish to allow, whether gay "marriage" or "enhanced interrogation," are suddenly made possible by the destruction of their opponents' definitions of words like marriage or torture.