If you can, your best bet is to simply explain that you don't find yourself to be a good fit for the prospective clients' event. You're talking about turning people away before a contract is ever even drafted. Your contract language doesn't really matter until you and another party sign it.
Of note, I don't think this is a contract issue. You will get many comments here, but none will be as important as the actual legal advice you get from a real lawyer. I would tread very carefully, here, and consult an actual lawyer so that you understand your exposure and how it relates to not only federal law, but your local/county/state laws, as well. You might as well say that you don't want to photograph couples whose skin colors don't match perfectly. You still have to be careful refusing to do business with protected classes. Doesn't make you lawsuit-proof, obviously. More importantly, there are going to be a lot of straight couples who will probably feel the same way. Anyone Googling your name will now plainly see this, and I can't imagine too many gay couples wanting to do business with you, at this point. In practical terms, you've probably just taken care of it right now.
