It’s a privately owned conveyance. If they want you to wear a polkadot t-shirt, neon green trousers and clown shoes in order to use it, then you either do that or don’t use their services. You can’t say “You can’t make someone bake a pro-gay cake” then protest about someone’s non-discriminatory conditions of service and remain intellectually honest.
To be in the audience of television game shows and talk shows they actually tell you which types of clothing and colors and designs that cannot be worn in the audience. Usually because certain patterns mess with the cameras. Polkadot t-shirts, I believe was one of them.