Joe Carter has written two well-reasoned pieces, here and here, on the strange spectacle of Christians (notably Powers and Merritt) in favor of the government coercing Christians to violate their religious beliefs and use their artistic skills and gifts at same-sex "weddings."

Carter's argument is powerful and worthy of consideration. I especially liked his caution of where consistent application of Powers and Merritt's views could well lead us:

Please stop arguing that Christians should be forced to violate their conscience unless you are willing to be consistent in its application. On this issue, what our culture accepts cannot be used as the standard. Fifty years ago, racism was tolerated while sexual sins were publicly denounced. Today, the situation is reversed. Many Christians (surprisingly, even some Anabaptists) are now willing to argue (or at least imply) that the state should be able to force Christians to serve at celebrations of sexual sin. Yet, these same people will likely balk at claiming that we should be forced to serve celebrations of racial sin.
If, like the Pharisees, you want to bind the conscience of all believers to a standard that is difficult, if not impossible, to support by Scripture, the least you can do is to argue for its broad application. Tell us that the white baker is not only obligated to serve a same-sex wedding but that the African-American baker is obligated to bake a cake for the Aryan Nation's national convention.
If you want us to follow your legalistic argument, then at least have the courage to follow it to all its logical implications.