There is only one way to stop this development: Get the government out of the business of marriage. Right now.
.. Conservatism has always been about preventing the power of government from invading the lives of citizens. Leftism has always been about using the power of government to restrict the behavior of others. It is time for conservatives to recognize the reality of their situation, realize the dangers inherent in their insistence on government interventionism and act quickly.
Getting the government out of marriage would mean voluntary lifestyle arrangements governed by contract -- a practice that has roots stretching back millennia. Religious people would not be forced by the state to approve behavior they find morally problematic. They would not have to worry about their children being taught about such behavior. Conservatives would be forced to rebuild a culture of marriage rather than focusing on a crumbling legal bulwark.
Conservatives lost the culture. Then they lost the law. They can only regain traditional values by removing legal coercion and incentivization from the table -- the left will never hesitate to use those means -- and focusing once again on the raising and production of children within a culture of traditional morality.