Thomas Messner at the Heritage Foundation explains how same-sex marriage threatens religious freedom by triggering nondiscrimination laws.
Here is the abstract:
Proponents of religious freedom have firmly established that same-sex marriage threatens religious freedom in a number of ways. In response, some have argued that certain threats to religious freedom discussed in this context have more to do with nondiscrimination laws than with the legal status of same-sex marriage.
This argument reflects certain realities. Conflicts between same-sex marriage and religious freedom will often involve some type of previously adopted nondiscrimination law or policy, and nondiscrimination laws can impose burdens on religious freedom even in jurisdictions that do not legally recognize homosexual unions as marriages. But neither point diminishes the threat that same-sex marriage poses to religious freedom.
Same-sex marriage is likely to trigger a number of conflicts between nondiscrimination laws and religious freedom that otherwise would not exist, and threats to religious freedom are no less troubling because they involve nondiscrimination laws and same-sex marriage, not just same-sex marriage...
From the conclusion:
Where nondiscrimination laws have been enacted without consideration of how they would interact with radical legal developments like same-sex marriage, lawmakers should reconsider and update those laws wherever appropriate. Failure to do so threatens to turn the marriage debate from a culture war into a conscience war and provides an additional ground for proponents of religious freedom to oppose the expansion of nondiscrimination laws in the future.