Religion takes part in political issues. For example in France, try banned the use of burkas (cloth around girls head) in public schools. This generated a lot of polemic and a lot of Muslims indignated with that new law. If religion had power in the government that would be bad, I just have to look to 1700's in my country.
:/ The reason for banning the burka had nothing to do with the Islamic belief IN that, it just so happened that it was related to Islam.
Religion should not influence the law, that is why gay marriage has nothing to do with your religious beliefs.
Honestly you'll learn that if two people love each other, its no crime to have them married. The limits on gay marriage are only there because people hated gays in the past honestly, just like in the US where white and blacks couldn't marry or w/e.