Well, they used to; I'm not sure that they do any more. Look at what has happened to the word "gay". Used to mean lighthearted and happy.
The word "gay" doesn't appear in enforce-able law to my knowledge, and Holy matrimony is a centuries old church practice governed by church law, and may be defined in church bylaw.
No law I know of can force a pizza maker to feature steak on their menu if that is not their cuisine or their competence.
Performing "gay marriage" ceremonies is one thing. A civil authority might be forced to recognize them. A church can limit what they do to performing ceremonies of holy matrimony as defined by application of Bible-based principle.
Perhaps it will help some of those getting married to focus on what the meaning of holy matrimony is in the first place and not to enter into it casually as some do.
FReegards!