Refusing service because of political party.

[johan]

You can now, and always could, refuse to serve someone simply because you don't like them. I think you're a jerk, get out of my shop. Perfectly legal. So this changes nothing as far as slippery slopes go.

Sure, in that case I don't think things should be that way. The cake guy can just say he doesn't like the gay couple, telling them to get out. This nullifies the whole idea of protected groups.

It does nothing of the sort. Protecting classes of people does just that, it protects the entire group from being discriminated against based on the class. But you can be a member of a protected class and also be an individual whom I think is an asshole. It is and should be illegal for me to refuse to serve all members of a protected class simply because I think one or more members of that class are assholes. But it is not and should not be illegal for me to refuse to serve any individual I find to be an asshole regardless of what protected class they may also be a member of.