As someone who does not believe the government should sanction gay unions, I will readily admit that religious belief is the underpinning.
However, I believe that God's laws were designed not to deprive us of pleasure, but to protect us from dangers, seen and unseen. Thus it to our mutual benefit for all of us to obey his teachings, even those who do not believe.
Of course, this must be balanced against the notion of freedom. Indeed, Jesus says that he will convict the world of sin because they do not believe in Me; so apparently, his priority is faith first, then behavior. And Paul informs us that we should indeed judge fellow believers more strictly on moral issues than non-believers. So there is a case to be made that the Bible does assume a certain civil freedom that we Christians should not necessary indulge in.
Nevertheless, I find it reasonable that our beliefs at least contribute to our opinions about what should be codified in law.