No, not about the idea that people shouldn't discriminate except where it is rational to do so. If that's the way that people want to be then so be it. Rather, we're confused about the fact that people keep calling for laws on this basis. Note that this nothing at all to do with things like Jim Crow: that was a series of laws to force people to discriminate. Or, if you prefer, it's everything to do with Jim Crow: for as Gary Becker pointed out the reason for those laws was the thought that in the absence of them then people would not discriminate in the manner that the racists thought everyone should. Showing that left alone people might well be able to rub along quite happily, even if not perfectly.
But we go a bit further than that in these cases of gay wedding cake refuseniks and the like. There's at least two possible reactions to that sort of discrimination. The first is obviously the law. But we're rather large believers in the idea that markets (and yes, social pressure and reaction is a market in this sense) are rather more powerful. To refuse to serve a potential customer because of race, gender, sexuality or any other such irrelevance is of course to be displaying a socially (in this society, the one we're in, in general) undesirable prejudice. And the question then becomes, well, what should be done about it?
Well, if it actually is a socially not desired prejudice being declared then we'd expect there to be some social and or economic consequences of it being expressed. People not using that supplier for example even if without any direct boycott being organised. That supplier going bankrupt as a result of not gaining custom perhaps.
Let us be serious for a moment: any pub which displayed the notorious no dogs...(insert prejudices of choice here) sign would be out of business within weeks. It's therefore not obvious that we actually need a law stopping people from posting such signs.
Another way of looking at the same point is that a society where it's possible to gain majority support for laws banning such signs is fairly obviously a society in which social and business pressures would stop people from displaying that sort of prejudice anyway. So we're left really rather wondering what is the point of the laws in the first place.