I would tell them that of course I have to serve them, but if I were to vomit at their ceremony, that could cause a problem for them. Of course it is their choice. :)
Better yet, take their money, then “call in sick” the day of the homofest. Of course, you’ll (eventually) have to refund their money but they will have to scramble to find a last-minute replacement and for reasons entirely out of your control.
If you take lousy pictures where you would normally perform at a much higher level, they can sue for breach. But you can’t help being sick, can you?
And to prevent their bringing suit for your failure to provide a backup photographer, make it clear that you are the sole resource and that any such provision will cost them extra. MUCH extra. And be sure to include a “force majeure” clause in the contract.
I wondered why syrup of ipecac was removed from store shelves years ago. They were planning ahead!