Doing business with whomever one wants, while denying to do so to others on whatever whim, is a fundamental tenet of freedom
That bullshit argument was rejected pretty soundly 50 years ago. It is reasonable in limited circumstances, for businesses which can only deal with a very limited range of customers. It is not considered reasonable for any business which claims to be open to the public--we decided long ago that you're either open to the public or you're not. You cannot be open to the public except for women; you cannot be open to the public except for blacks or latinos. Etc.
While a business shouldn't be allowed to not serve a segment of society, a business shouldn't be forced to contribute to something to which they object (on any grounds, but religious grounds for this argument). So while a bakery should have to sell a pre-made cake/cookie/whatever to any customer that walks in, it shouldn't have to make a cake promoting a gay wedding or a NAMBLA meeting or a Jihad Dance Party or Furry Orgy (I'm not equating those things, I'm just listing things that many people would object to being a part of). In an extreme example, a Jewish-owned bakery shouldn't have to make a cake with a swastika or "Death to Jews" written on it. Some people would see making a cake with a rainbow on it for a gay wedding as just as offensive. Let them believe that and take your business elsewhere - why would you want to give them money in the first place? Bring attention to that business, boycott them, do everything legally possible to embarrass them, but don't force them to go against their beliefs, no matter how wrong you think that those beliefs are.