The basic strategy here is to put the prospective client off in a legitimate way so they don't call back, or have them disqualify themselves without the vendor having to weigh in explicitly regarding his willingness to participate in a same-sex marriage (SSM). I suggest the vendor follow these steps.
Don’t initially answer the question, “Will you perform a SSM service?” or “Will you provide your floral/baking/photography service for a SSM?” Instead say you’ll need to get some information first before you can go any further. Have a basic form you fill out in your first conversation with all customers listing the full name of each “wedding” participant, the name of the person officiating the event and the event location, if they have that information. You don’t need to tell them this at the time, but your goal is to find out if the request is legitimate, or just an attempt to legally corner you and provide grounds for a lawsuit (it happens). Once you’ve gathered the relevant information, tell them you’ll need to check out the details, thank them for the call, say, “Goodbye,” and then hang up. Do not say you will call them back. If they resist, tell them you simply cannot go any further with their request unless you have the basic info. If they persist, simply thank them for the call and hang up. The form should be filled out for every potential client, since they may not reveal up front that their request is for a SSM. You will only be able to determine this by getting the names of the bride(s) and/or groom(s). You may even need to ask if this is a same-sex wedding or a heterosexual wedding if you’re not sure, but say nothing that might indicate your own views. Next, follow up to see if the request is legitimate. If not, you can simply ignore future requests from this party, telling the caller that according to your research, this was not a legitimate request. If you discover it is legitimate, with luck the client will not call back, having chosen other vendors who are not so difficult to work with. If the client does call back and it’s a legitimate event, move to step two.
Ask the clients what they are looking for in a vendor. Do they want someone who is excited for them, who will celebrate with them, and whose support and approval of the event will naturally be reflected in their work? Of course they would. If that’s the case, tell them that you would not be their best choice because you could not bring any of that to the service you would provide. You do not agree with these kinds of events, you have religious compunctions about them, and it would injure your conscience to participate. Ask them, “Do you want this kind of person serving your wedding?” Do not say, at this point, that you refuse to participate in the event, though. Instead, give them plenty of room to disqualify you themselves. If they do, you’re off the hook. If not, proceed to step three.
This is where each vendor has to decide for himself what to do. If he cannot, in good conscience, serve an event like this, then he should decline. The risk is that such a decision could expose him to legal risk. If serving the event violates only his tastes and subjective sensibilities, but would not be sin in his judgment, then he might want to do the event in spite of the discomfort and save himself any legal hassles.