Skatje Myers, daughter of the (in)famous PZ Myers, and self-described “atheist, agnostic, rationalist, complicated vegetarian (sans dairy and eggs mostly, avec dumpster diving, by-products, and ethical sources), existential and moral nihilist, social democrat,” explains what is and isn’t meant by the disestablishment clause of the American Constitution.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof
The religious clauses of the first amendment serve three purposes: The government may not set up a church (as England did), the government may not create laws that unfairly support one religion over another (for instance, taxing all religious institutional except the Christian ones, or Congress deciding to put crap about God on government currency), and the government may not prevent anyone from practising their religion.
Enacting legislature that lines up with the moral views of a religion (“Thou shall not murder”, anyone?) generally does not violate this. It does not favour that particular religious establishment, it does not say “you have to follow this religion” or “you can’t follow your religion”. On the other hand, we’d have some problems with “Thou shalt not have other gods before me”, but that’s not the sort of legislature we’re talking about here. A Christian who does not want gays to marry is not asking the government to force everyone to be Christian; they are asking the government to force gays to not to be allowed to marry. The Constitution allows for this.