Wow...
No matter the business, no one has the right to my products or services. Refusing service is the given right of any business owner for whatever reason (s)he sees fit as a part of the risk (s)he took in opening said business. Yes, there is inherent risk in exclusion, but for most Christians, that is the price of living in holiness.
Serving sinners - I would think that customers coming through the door looking for a cake that says, "Thanks for being a great mistress and filling in when my wife cannot." Would probably be shown the same courtesy as the individuals looking to get their same-sex marriage cake made.