“Marriage and religious freedom will stand or fall together.”
So says Matthew J. Franck from the Witherspoon Institute in a very thought-provoking article at The Public Discourse Same-Sex Marriage and Religious Freedom, Fundamentally At Odds.
Franck concludes:

The transformation of the law to redefine the meaning of marriage will be bad for marriage, bad for children, and very bad indeed for those people of faith who want to maintain their faith’s teaching on marriage, in their religious institutions and in their work. The preservation of meaningful religious liberty, it turns out, is inseparable from the preservation, in our legal order, of the truth about marriage. They stand or fall together.
There is no shortage of cases to suggest that freedom of religion will be threatened by redefining marriage.
Redefining an institution which is thousands of years old and predates government is problematic for all sorts of reason. But one of the most obvious problems is the attacks on freedom of religion, freedom of conscience, and common sense which follow.