"Listen," he said. "The way I look at it is if everyone were gay, society would cease to exist. Therefore, it can't be what's right."
Over the years I've heard this argument from many people in one form or another. As exhibit A, take page 19 of this document purportedly penned by Elder Dallin H. Oaks.
"One generation of homosexual 'marriages' would depopulate a nation, and, if sufficiently widespread, would extinguish its people. Our marriage laws should not abet national suicide"(I obviously have no personal knowledge of this document's authorship, but methinks it reads quite a bit like Elder Oaks.)
To my roommate many moons ago, I believe my counter went something like, "Yeah, but if we all became accountants, society'd be pretty hard up a generation out." We laughed. the issues with my argument obvious. But thinking about it I believe there's a sliver of truth in it.
Today, if someone made this argument, I think I would respond, "Yeah, but if everyone practiced celibacy we'd be no better off one generation out. And yet that's what we ask of our homosexual brothers and sisters." So if the one argument against homosexuality is sound, it stands to reason that celibacy is wrong too.
A concluding note: I want to make very clear, I am not arguing that homosexuality is right nor that it is wrong. I'm merely pointing out that the "if everyone did it" argument doesn't hold water. The morality of homosexuality must be decided on different terms.