Sure, this position can exist. It simply doesn't reflect the status quo or have much to do with whether the state should expand marital privileges to cover homosexuals. This is basically an entirely different and opposite campaign: "Stop encouraging permanent coupling all together" instead of "please give more people special encouragement to couple permanently". Also, this argument is what is sometimes meant by "gay marriage will destroy marriage" -- you're extrapolating that based on the contentious attitudes around the gay marriage debate, marriage should stop being a legal thing. This is pretty much as close as you can get to "destroying marriage" in an objective or technical sense.
>But if you want special privileges from the government (like tax breaks) for your religious status, then you need to accept everybody's definition of marriage.
wut. If religions only qualified as religious institutions if they accepted everything the government said, the only religion that would exist would be "nationalism".
Further, from the referenda that keep saying "no" to gay marriage, I think "everybody's definition of marriage" may be better aligned with the predominant religious view anyway...