I can’t imagine the Supreme Court going against its previous permission to allow gay marriage. Has the Court ever allowed something significant like interracial marriage, gay marriage, sodomy, or the like and subsequently upheld a newer, more restrictive law against what was previously deemed constitutional?
When the issue came before the Supreme Court, there was a split of opinion; the majority wanted to deem homosexual unions as marriage, and the minority would either have prefered to leave the situation as it was (which included some States acknowledging marriage-like rights and responsibilities such as inheritance and other related spousal-tyupe matters, such as health issues) or would have preferred to call them civil unions, thus bringing some consistencies in state laws.
Stare decisis is not an absolute rule of law but in most circumstances is followed. That makes it highly unlikely that the case law establishing a legal right to marry extends to same sex couples.
Antonin Scalia foresaw the conflict this created, but could not convince enough of the judges to either address the 1st Amendment issue, or to provide some means of indicating that the 1st Amendment still was in force.
As I have not made a point of following any subsequent Court decisions, There is still mischief afoot in some quarters.
The baker (who was not Catholic) in Colorado fought to the Supreme Court and was vindicated, but the Court in that case did not treat it as a 1st Amendment case, but rather treated it as a “workproduct of an artist”, which was a weasel way to avoid confronting the isse which Scalia predictied. Last I heard, Colorado said (something exceedinly impolite) and went after that baker again, and last I heard it was on appeal in a federal court.
The baker was not the only one to face the issue of discrimination. There was a woman florist who had a repeat client (man) for whom she had made numerous flower arrangements. He was getting married, and she also out of religious convictions did not want to make an arrangment for the wedding, and he brought a case against her also.
There may have been other cases; again I am not tracking.
And even if this is a rare matter, it is clear that if an individual who has a business which can be related to weddings (e.g. photographers, bakers, florists to name a few), their religious choice to not be involved may result in a Pyrrhic victory as they face the power of the State and the costs of litigating any actions taken against them. Lawsuits can be notoriously expensive even if you win.
As to the moral side for a Catholic, I have not researched it but would suspect that it (providing whatever for the wedding) would be considered too remote to indicate many immorality on the part of the business person.
As an aside, this is not limited to the United States; the Knights of Columbus received a request, I believe in British Columbia from a gay couple wanting to rent their hall; I think ultimately they won - but the point is that this is not a US phenomenon.