Actually, you have a valid point. I could put up a sign that says “God Hates Homosexual activity” that would only show up when homosexual couples are checking out the facility.
Or something like that...
or have the officiating preacher go into an hour long (or longer) sermon about the evils of homosexuality, and then at the end the preacher could say “I know you two would like me to declare that you are married, but because your attempted union is an abomination In the sight of the Lord, I’m afraid I can not sanctify something so unholy if you wish to bring in your own fake ungodly ‘minister’ to conclude the ceremony, you are welcome to, but everyone else here will be leaving before you do so that they don’t defile themselves along with you- have a nice day and thanks for playing”