Question, do you really want "In God We Trust" on U.S. coins? Do you really want to associate God with the U.S. government that constantly goes to war when it's unnecessary? Do you want to associate God with the U.S. government that creates laws you disagree with? For example, if you're pro-life, do you want "In God We Trust" plastered on a coin belonging to a government that is pro-choice?
I believe in Separation of Church and state because a government that forced people to believe in God or to live by God's morals defeats the whole purpose of letting people choose whether they want to believe or not.
If we don't base our laws off of God, then how will the gov. determine right and wrong? I believe the answer is the non-coercion principle. If gays want to get married, they can get married outside the Catholic church. Sure, you may think it's blasphemous, but it's their choice to decide whether they want to go to heaven or hell.