Let me clarify something. I make a sharp distinction between a legal concept of marriage and a moral one, or a religious one. There may be reasons for governmentally defined marriage to include marriages that might fail to meet moral standards. So even if someone accepts a moral case against gay relationships, this does not guarantee that the government shouldn't recognize such marriages. You need something else.
This has the support of one who has a lot of authority in the Christian tradition:
After Jesus had finished saying these things, he left Galilee and went southward to the region of Judea and into the area east of the Jordan River. Vast crowds followed him there, and he healed their sick. Some Pharisees came and tried to trap him with this question: "Should a man be allowed to divorce his wife for any reason?" "Haven't you read the Scriptures?" Jesus replied. "They record that from the beginning `God made them male and female.' And he said, `This explains why a man leaves his father and mother and is joined to his wife, and the two are united into one.' Since they are no longer two but one, let no one separate them, for God has joined them together." "Then why did Moses say a man could merely write an official letter of divorce and send her away?" they asked. Jesus replied, "Moses permitted divorce as a concession to your hard-hearted wickedness, but it was not what God had originally intended. And I tell you this, a man who divorces his wife and marries another commits adultery--unless his wife has been unfaithful."
This seems to me to suggest that the law can be justified, for its own reasons, in accepting relationships as marriages which are, in the final analysis, not fully moral.
Here is C. S. Lewis on the legal-moral distinction.