“The issue is that some people believe the 1st amendment says you can’t tell someone else HOW they should practice their religion."

And yet that is exactly what the business owner is doing to the customers responding to his freely made invitation to come buy from him - refusing a customer because their beliefs allow them to have marriages that the owner’s won’t allow him to have for himself.

Once the invitation to the public is made its too late to hold them to any religious standard since they have a right to have a their own standards.  The owner knew he couldn’t discriminate against a customer because of a civil rights class before he made the offer.

Can’t sell wedding cakes to public regardless of their civil rights class don’t offer wedding cakes to the public at all, that’s what private membership organizations are for.