Government has no rightful role in marriage. People cannot marry animals, and people cannot marry others of their own sex. But it has nothing to do with government. People are married by virtue of promises before, and to, God. Not because some government says they are.
Actually, it does. Govt. has a role in keeping society stable. The foundation for doing that is the family with a father and mother. Therefore the govt. should promote marriage between mother and father. Since homosexuals can't have children they have no reason for marriage (other than to try and normalize their deviant behavour).