I still don't get this argument...

In Macleans this week, some guy named Stanley Hartt (who apparently worked for Mulroney) makes this argument on same-sex marriage:

But here's the rub: to millions of Canadians, the concept of same-sex marriage is terminologically oxymoronic. Their status as married persons is something they elected, often in the context of a belief system, intending it to emphasize a specific social status. They are not reactionaries or rednecks or insensitive or out of date, and resent being cast as such. They are generally hard-working, law-abiding, taxpaying, God-fearing people who believe there is no need for social progress to be made on their backs.

What exactly does Mr. Hartt mean by this? I still don't get how same-sex marriage constitutes progress (or even regress, if that's your view of it) on anyone else's back? I said it before and I'll say it again, I am straight and someday I may just get married, and I cannot, for the life of me, figure out how gays being married will somehow impinge on my own marriage. How does the fact that gays and lesbians might do the same thing hurt any future marriage of mine? What's more, this argument is being made by conservatives! I'm no expert but I thought that conservatives were big on, you know, personal responsibility and individual autonomy and all that. But no, not with marriage, according to Hartt's argument, the minute any gays get married, all hetero marriages are in grave danger of flying apart due to the centrifugal forces of gayness. There is nothing that straight couples can do!

Maybe I've misunderstood what he's saying, offer me a better understanding of it if you can.