Anti-discrimination laws are discriminatory themselves, in that they dictate who I must do business with, even if I don't want to.

Can a restaurant discriminate based on race, sexuality, gender, religion, or any other reason?

Legally, no, you are correct. But I disagree with the law, like many others. But that doesn't mean I agree with the actions of racists, homophobes, and others who would decline serving others based on these attributes. It's a slippery slope when you advocate that someone must serve someone else. Would you say that the Jewish baker should be forced to make a Nazi cake? An American patriot baker should be forced to make an ISIS cake? I understand those are extremes, but this isn't a case by case ruling. It must be consistent across all levels, otherwise it's not a principle, but rather a preference.

At the end of the day, it comes down to freedom of dis-association. No one can compel me to talk to, work with, sell to, or anything else otherwise, to anyone I don't want to.