<We are not talking merely about selling wedding cakes but cutom made wedding cakes.>
Yes: sold in accordance with the law, treating all customers the same.
<The seller has the right to refuse to design a cake which offends his conscience.>
Of course he does; but if doing so violates the law, he must accept the consequences of the violation. 
I think there is a work-around available for a baker who is determined to discriminate against same-sex couples.  Such a baker could design a specific number of wedding cakes that do not violate his conscience, and his customers could choose from among them them.  However, if the baker is willing to personalize those designs for any customer, he must be willing to personalize them for every customer.
In my experience a determined, persistent bigot is nothing if not determined and persistent.