Alright, here's the reality of the land:
America was founded on Englightenment Principles (the ideas in our Constitution are borrowed from the French Constitution and inspired by Englightenment Age philosophy on liberty, freedom and justice).
There is a radical Seperation of Church and State. Church cannot regulate religious beliefs as laws, and the State cannot regulate religious beliefs. This is mutally beneficial, it provides a safe, bias-free (in theory) system that does not make judgements based on religious beliefs. Imagine if a Buddhist was being sued in court, and he was judged guilty for not worshipping the Judeo-Christian God.
Religion should not mix in politics. If you start regulating laws based on religious morals, where does it end? Whose religion should the laws be based on? If you say Christians, what kind of Christians? If you say Protestants, what kind of Protestants? If you say Conservative Protestants, which denomination or sect? If you say laws should be made based on the Bible, which interpretation?
Should we ban wearing polyester or multi-colored suits? (it is considered "sinful" in the Old Testament along with "homosexuality"). Should we ban pork and shellfish? Should we enforce that everyone attends Church on Sunday?
If you say that all religions should be open to influencing law, should we change "under God" in the Pledge of Allegiance to : "One Nation, under God/Christ/Yahweh/Allah/Brahman/Buddha/Shunyata/Dao/Goddess/Ahura Mazda/Lucifer/"? If not, then why?
If we let Christian beliefs influence policy on Gay Marriage and Abortion, why not let it influence other things too? Why not enforce Jesus' policy against interest? (Jesus said to not charge interest on loans)
Marriage is a federal issue, not a religious issue. Mariage is a federal issue because:
1) Seperation of Church and State
2) Christianity did not invent marriage
3) Non-Christians get married also
That's why you need a MARRIAGE LISCENCE in this country for the goverment to recognize your marriage. If marriage was solely a religious issue, being married in a church would be good enough and you would not need a liscence. That's also why people can get married outside of churches and have their marriage recognized.
Abortion is also not a religlious issue because the laws on death in this country are not influenced by any religions (due to Seperation of Church and State). A woman's right to choose is a Constitutional right due to the First and Fifteenth Amendment.
We shouldn't legislate "morality" on religious basis because our laws are built off of Englightenment Age secular morals in the first place. To regulate "morality" based on any religion (especially on a single religion) would be stepping into the realm of theocracy.