I'm more a freedom of association guy. I don't like the kind of preferences that would have the celebrant turn down a gay couple, but I don't much think the government ought be forcing people to take clients. I know that mine is a minority view. I still hear Walter Williams's lectures on discrimination in the back of my head: how he discriminated when he chose Mrs. Williams over all the other women who wanted him and how nobody could have forced him to date a selection of women that reflected the diversity of America's ethnic composition before making his choice.
The bit I really really don't get is why anybody would want to have their wedding day ruined by having an officiant there who hates them or who hates that they're being married. What kind of utility function gets more joy from the discomfort of someone forced to officiate over them than pain from having a horrible awkward wedding day?
We had no officiant at our wedding (more below). But had we been forced to have one, and we found that the officiant we'd picked hated Canadians and thought it a sin that Canadians be allowed to marry Americans, we'd have been looking for another officiant rather than a lawyer. I can understand the utility of making a political point. But getting smugness jollies by having someone who hates you and your marriage preside over your wedding? I just don't get some folks' utility functions.
When Susan and I married, we had no officiant at all. Pennsylvania law lets non-Quakers use the Quaker-style ceremony. The Quaker church hasn't a hierarchy, so weddings there simply formalize the community's recognition that a couple has married. So everyone in attendance at our wedding signed on as witnesses to our wedding contract; Susan and I officiated our own ceremony to which representatives of neither god nor government were invited. Instead we were married on the recognition of our friends and family - people we like, who like us, who we wanted there and who wanted to be there. Self-uniting marriages are recognized more broadly than Pennsylvania. I don't know why more libertarian atheists don't use them. You still need to get the marriage licence authorized by the state, but why invite the state to the ceremony if you hate the state?