I think it's about belief here, not skin color or genitals you crave.
In this case, the baker says "I'll serve anybody anything except specifically wedding cake to same sex couples."
Would you force a black baker to cook a Confederate flag cake? Make a Muslim baker bake a cake saying "Mohamed was a pedophile"?
Same logic applies.
"confederate supporter" is not a protected class and it is totally legal to discriminate against that characteristic of a person
this stuff is not hard to understand