I think it's clear; if you're running a business, you
shouldn't be allowed to turn would-be customers away and say "it's because you're black" or whatever. That's not good for society, to permit that kind of casual discrimination. We've been down that road, and things weren't getting better until laws started being passed.
I mean, come on, if you're such an asshole that you really, really need to discriminate against people for whatever reason, then say "sorry, we're booked up". Otherwise, make the stupid cake, whether the couple is gay, or mixed-race, or whatever. And if you can't do that, don't start your own business.