I don't think they should be forced to sell a cake promoting a political/religious view that they don't agree with.
They would have served them without issue if it didn't have the specific message on the cake so you can't claim they are refusing to serve them because of their sexuality.
No. They refused because they disagree with homosexuality, not because of the message.
Refusing to make a gay wedding cake is discriminatory - how is it equitable that hetero couples can get a service that homosexual couples can't?