This is the current favorite argument of the religious right in the battle against gay marriage.
Let's dissect this a bit.
First of all the argument isn't about gay marriage per se but rather some of the implications resulting from making gay marriage legal.
The argument actually focuses on the question of who can refuse service to who and for what reasons.
Clearly a vendor or store owner can refuse service for specific actions. This is the "no shoes, no shirt, no service" argument and the reason why a Jewish store owner can refuse service to a Neo-Nazi. It's not unknown for store owners or restaurants to eject rude or rowdy customers. As a society we accept this sort of thing. As a matter of fact we actually applaud it.
At the other extreme we do not condone refusing to serve blacks at a lunch counter simply because they're black. As a society we have decreed that you cannot refuse service to someone because you don't like who they are. This includes race, religion and national origin.
In some states it also includes gender orientation and gender identity. In other words just like you cannot discriminate against someone simply because they are black, you also cannot discriminate against someone simply because they are homosexual or transgender.
Fifteen states prohibit discrimination based upon gender orientation and gender identity. An additional six states prohibit discrimination based upon gender orientation only. Ten other states have partial protection based upon gender orientation.
The other nineteen states have no specific discrimination protection for homosexuals or transgender people.
So, what does this have to do with gay marriage?
The question becomes is gay marriage a specific action or simply a part of who someone is.
Some Christians believe that gay marriage is immoral and that providing any sort of support for a gay wedding, flowers, a cake, photographs, is essentially condoning an action which violates their religious beliefs. Clearly they consider it a specific action.
But then the problem becomes why limit the objection to gay weddings? Gays who aren't married also most likely engage in specific actions some Christians would consider to be against their religion. So does that mean by extension someone can refuse service to someone simply because they're gay?
Do you see the conflict here?
Let's ignore that for a moment and focus on gay marriages. Can someone who considers a gay marriage immoral refuse to supply services for a gay wedding? In the states that prohibit discrimination based upon gender identify the answer is apparently no you cannot.
The two most well known cases of store owners getting into trouble because they refused service to a gay couple were in Oregon and New Mexico which are two of the fifteen states that prohibit discrimination based upon gender orientation and gender identity.
As for the states with no prohibition in place, I assume one can discriminate with impunity UNLESS there is some other law or constitutional provision that might apply. That would probably take a court case to determine.
Gay marriage becoming legal everywhere isn't going to change this much other than the simple fact that if gay marriage wasn't legal then no one would be asking anyone to supply services for a gay wedding.
Personally I would be willing to allow store owners to refuse services for gay weddings if they're willing to post a clear sign to that effect. If you're sure you're right then this shouldn't bother you at all and it would prevent the awkwardness of a gay couple trying to get services and then being refused.
Let's see how this one goes over shall we? Of course this might lose you other customers as well but why should anyone care about that when their eternal salvation is at stake?