Do you don't mean that we can't pass a law that would forbid a religious practice? I don't think so. We have all kinds of laws that forbid polygamy and incest, so why couldn't we pass others that would forbid similar practices?
And I don't think we are a democracy. Democracy is just one of the mechanisms we have for making decisions. The democratic process is subservient to the prohibitions defined in the Constitution.
We have civil laws against polygamy. This is equally inconvenient to Muslims, Pagans and certain splinter sect Mormons. The law prohibits a certain action- taking more than one wife. This is because- and only because- a majority of Americans think this should be the law.
If a sufficient majority of Americans ever decide that that particular law needs to be changed, it can be- that's democracy.
And the incest laws have a valid basis. The State has an interest in protecting public health- in this case, protecting unborn children from genetic disorders resulting from inbreeding. This makes no mention of religion.
And as much as I love and treasure the Constitution, the will of the people is NOT subservient to it!! How many amendments are we up to now?
No, we are not a pure democracy- that's mob rule. We are both a representative democracy and a constitutional republic, with a unique system of checks and balances that has kept us more free and more prosperous than any other people on earth for over 225 years. The fact that this diverse nation of rebels, outcasts and malcontents from all across the globe is still holding itself together is a wonderful thing indeed.