For me it is simple. Nobody, for ANY reason, whether they are black, white, polka dotted, straight, gay, leftwing, rightwing, Christian, Muslim, Buddhist, Jew, or an accordian player, etc., should be required to provide a product or service or participate in any way in an event or activity to which that person objects and would not normally carry. For a person to have the right of their own conscience in that way discriminates against nobody so long as the the products that the person normally carries are sold to everybody who wants them.
The Jewish baker should sell the cupcakes he has to sell to the Nazi who comes into the shop to buy them. He should not have to accept a special order for cupcakes with swaztikas or I hate Jews on them, however.
The gay baker should sell the cupcakes he has to sell to the Westboro Baptist member who comes into the shop to buy them. He should not have to accept a special order for cupcakes decorated with Gays go to Hell or cater an event at the Westboro Baptist Church.
The staunchly pro life baker should have to sell cupcakes he has to sell to the Planned Parenthood staff who comes into the shop to buy them. He should not have to accept a special order for cupcakes decorated Pro Choice or cater a Planned Parenthood Event.
The Christian baker who cannot believe in same sex weddings should sell the cupcakes he has to sell to the obviously gay couple who comes into the shop to buy them. He should not have to bake a cake decorated for a same sex couple or set the cake up at their reception venue.
Rule of thumb. If you want to buy what I have to sell, you are welcome to do so. If you want me to accept a special order for something that offends me, I should have every right to say no.