Until recent times, theists have been able to rely on the public to oppose gay marriage for religious (God says no) or just-so (It’s just wrong) reasons. Now that many religious people have come to accept gay marriage either because they see nothing wrong with it or because they do not want to impose their religious beliefs on others, theists who oppose gay marriage have lost much of their support. And now that people in general–and young people in particular–expect better arguments than “just because,” these theists have had to try to engage with the secular world on its own terms. This means identifying the harm that gay marriage does to individuals, families, and our society; showing that said harm outweighs the good of gay marriage; explaining why other unions or behaviors that may do harm are not illegal; and so on.
They clearly struggle with this line of reasoning, often bringing up wholly unrelated matters (divorce) or technically unrelated ones (adoption by gay parents) and citing dubious studies with results that are not consistently reproduced. This is to be expected from people who took for granted that people would continue to accept their baseless moral code. But I want to call attention to another popular argument, often used in conjunction with the naturalistic fallacy, that goes something like this:
Premise 1: The human race ought to continue to exist (i.e. to reproduce).
Premise 2: If we were all homosexual, we would be unable to reproduce.
Conclusion: Therefore, homosexuality (or at least homosexual behavior) is wrong.
Of course, the conclusion is used to condemn gay marriage as well.
In terms of logical validity, the argument is fine. But that is literally the only good thing that I can say about this argument.
The idea that evaluating the consequences of a behavior if everyone engaged in it is proper moral reasoning is absurd. Consider: if everyone decided to become a plumber, no one would be able to do anything else. But we do not therefore say that no one should become a plumber.
Or consider an analogy that might hit a little closer to the theist’s home: if everyone decided to become a priest and thus be chaste, the human race would die out as surely as (in fact, more surely than–see problem 2) it would if everyone were homosexual. Does this mean that there should be no priests?
Premise 2 is false: reproduction requires neither heterosexuality nor sexual intercourse. As long as a sperm meets an egg and the resulting zygote develops in the proper environment, we will produce more humans. If we were all gay and we wanted our species to continue to exist, we could easily make it happen.
No matter how popular the first premise might be, it is not a given. If humanity died out, there would be no people to mourn its loss. Our species is not an entity capable of thought, feeling, or desire anyway.
This is the quality of secular argument that one can often expect to get from someone whose primary motivations are religious. When asked why murder or theft or fraud is wrong, I suspect that most people would first appeal to harm, not God or the soul. So when someone first appeals to God or the soul in support of his position on an arguably less obvious moral issue, it should be no surprise when his secular arguments are poor.