They aren’t denying anyone anything. They marry one man to one woman. Same-sex marriage is not a service they offer.
It is like me suing a car dealership for refusing to fix my TV.
You are too right. This couple advertised “one woman one man” marriage. Saying they have to marry something other than advertised is like forcing a VW auto mechanic to fix Fords also. Or forcing a Baptist to sprinkle as well as immerse.