It isn't that one shouldn't bake cakes, it's that one shouldn't bake cakes for weddings or other clients that you find religiously offensive. In other words, if you engage in a public business, you should have the right to refuse service to potential clients based on your personal religious beliefs about the client and their traits and behaviors (beyond what is reasonably allowed for public health and safety).
I think this question was prompted by the Religious Freedom Restoration Acts of the various states (The Indiana version got quite a lot of publicity around NCAA finals time), and the US version, which create a right for individuals in private business or in government agencies (as is happening in Texas right now, with county clerks refusing to issue marriage licenses, with the vocal support of the state's attorney general) to refuse to perform their jobs on the basis that the individuals' beliefs prohibit participation in the offending activity--such as a same-sex marriage--and exempt them from lawsuits and government enforcement of laws. The common example used in Indiana was that a baker could refuse to bake a cake for a same-sex wedding because it offends their personal religious sensibilities. A pizzeria in rural Indiana publicly declared that it would decline to cater any same-sex weddings, and apparently received hundreds of thousands of dollars in donations in support, although there was essentially no chance of a lawsuit or any significant repercussions economically from the declaration.