This argument is old and misapplied.
Marriage has three aspects.
1. The Religious - The government has nothing to do with this. Gay people can, and do, go to churches and get Married without the government's approval or "blessing", nobody is getting arrested for this.
2. The "Moral". You make a promise, you either keep it or not. Gay people can and do make these promises and keep them daily.
3. The "Legal". The government "endorses" through recognition certain specific relationships based on what? They can't base it on religion.
IMO, the government shouldn't be interfering, either through "recognizing" or through direct negative action (arresting and charging), in anything that does not have a victim. There is no direct victim of "gay marriage" so long as they are adults.
Seeing as it is factual that gays get married daily, it is stupid to reject simple recognition based on weak arguments. Let people do what they will, it really isn't any of our business so long as they are both consenting and both adults.