Thomas Berg, writing in Christian Century, discusses the issue of religious liberty and marriage equality:
Recent advances for same-sex marriage have raised important issues concerning religious liberty. The four New England states (Connecticut, Maine, New Hampshire and Vermont) whose legislatures recently voted for same-sex marriage have all had to consider how broadly to protect churches, organizations and individuals that object to participating in or facilitating such marriages. A national TV ad against gay marriage sounded alarms about religious liberty and triggered parodies from gay rights groups (and Stephen Colbert).
The issue of religious liberty is significant, but it is also manageable. A state that decides to implement same-sex marriage can and should craft provisions that protect religious dissenters without erecting any substantial obstacles to gay couples marrying.
Although clergy persons would not be required to officiate at same sex weddings there are a host of other questioned raised in the provision of services by religious institutions to same sex couples. Read more here.