What if your religion says that "black people are inferior to white people?"

This is at the very nature of the debate: Are people of different sexualities HUMAN rights, in EXACTLY the same way that people of different ethnicities are.

Because if they are, civil rights supercede religious rights. ("Your fist's freedom ends where my face begins")

Time and time again the US falls on the side that "no, gay people are not entitled to civil rights", diametrically opposite from every other western nation.

That would have nothing to do with characteristics of their marriage, that'd be simple racial discrimination. The marriage would be an arbitrary detail in that scenario. I don't hold the same position as him, but if the guy genuinely believes he'd be punished in the afterlife for celebrating a same-sex marriage, then leave him alone. A wedding cake is not a basic human right.

OK, can I refuse to sell a house to a gay couple who's getting married, then? That's also celebrating their marriage, in a much more meaningful way.

It's exactly the same. "The cake was meant to be a celebration of a Black marriage, and the plaintiff's religion doesn't accept Black people as human."