To start, I believe pre-marital sex should not be made illegal. Now to play devils advocate.
First off, the government is well within its right to place a ban, considering the only piece about religion in the constitution is that you are free to believe anything you wish and that government cannot show favoritism towards one religion (This is in reference to things like putting up a nativity scene outside city hall). The "Separation of Church and State" was thought of by Tomas Jefferson and is nowhere in the actual constitution. Also the government will let you believe whatever you want, but will not let you practice it necessarily. You can believe animal sacrifice is okay, but try to practice it and see how that argument stands up in court.