As a business they are well within their rights to choose not to provide cakes with hateful or racist language on them. The closest comparison isn't a wedding cake for a gay wedding, its a cake with hateful language about straight people or some ethnic group. That's not the same as a wedding cake.
But if the baker makes penis cakes for bachelorette parties, then the baker has to make penis cakes for anyone. This isn't an unreasonable standard, this isn't asking a business to be superhuman. But the business has to be agnostic as to their customers.
Here.. maybe this is easier. Instead of all of these hypothetical situations involving gays, imagine that they involved fat or ugly people. Could a baker refuse to make a wedding cake because the bride was too fat or ugly?
Matthew 10:34Think not that I am come to send peace on earth: I came not to send peace, but a sword.