Lets reverse it a little, what if a gay couple owned a restaurant and they saw a two priests sit down at a table and asked to be served and the owners knowing these people are against gay relationships refused serving the priests and asked them to leave.
You are still on the wrong track, one more time:
Gay marriage is an event/activity, a thing you do. The point is that a business owner is free to think it good or bad and decide, based on that equation, whether he wants to be associated with it. If he decides against, he is not denying or rejecting a person.
A gay catering company would be totally in their right to not cater a Catholic event, get it yet???