In the US, can a caterer refuse to cater an interracial marriage?

Well, any laws can theoretically be abused by dishonest plaintiffs. Sorry that it happened to you, but on balance I feel that the social benefits of having anti-discrimination laws outweigh the costs.

AFAIK political expression is not protected anywhere, so I don’t think this woman would have had any legal recourse had she been thrown the fuck out of there.

Which, I think it bears noting, is about the whitest possible thing you can say.