This is why religions fight so hard against gay marriage. Marriage is a religious ritual. If you allow the government to dictate marriage, then you now allow the government to dictate baptism, temple ordinances, Muslim prayers, etc.
You have to keep church and state separate.
p.s. that being said, anyone that isn't for "civil unions" is a bigot.
The government is already dictating marriage. They perform marriages and divorces. They set the laws governing both. If your policy on civil unions is sincere then you should be against marriage performed by government. Marriage should be done by religious denominations only and civil unions by the state.
If marriage can only be done bya religious denomination than there is nothing stopping the LGBT community from establishing a church of their own and getting married.