You repeatedly express the belief that it's okay to discriminate as long as you don't accept public funds (in certain situations, granted). I don't agree with that. I believe that if you offer a public service, you should not be allowed to discriminate, regardless of your funding. ALL businesses receive benefits derived from public funds (roads, fire protection, etc etc), so I don't see why public-fund CASH should make a difference.
I get that if you are offering an essential service. Like a pharmacy, or grocery store. But a wedding cake? It's very much a discretionary item. I could equally feel indignant that a local boutique won't stock my size, but ultimately I'm not going to unduly suffer just because I have to shop elsewhere. If I was unable to buy contraceptives in the only pharmacy for 100 miles or more (and this has happened in rural Australia) then that's a massive issue.
I hear what you're saying, but I still disagree. Discrimination only on non-essential services is still discrimination. To use your example (AFAIK size is not yet a discriminatory quality) - If the only reason a local boutique doesn't stock large sizes is because they don't want fat customers - I don't think that's fair or right.