This argument presupposes that these judges are for gay marriage, as opposed to impartial arbiters of the law. Perhaps the judges genuinely feel that preventing gay marriage is unlawful or unconsitutional, and are willing to set aside their own personal objections?
Personally, I don't see how gay marriage is a threat to heterosexual marriage. Are there some poor heterosexuals that will be prevented from marrying? Are the gays cutting into some unknown marriage quota? Otherwise, it seems there's nothing needing defending..
Now, it isn't clear from your comment as to whether you yourself are for or against gay marriage, so I'm not trying to direct any criticism at yourself, but you don't know me, so I thought I'ld attempt to make that clear.