There's no formal "contract" between a shopkeeper and a customer. But if the shopkeeper advertises a good for a certain price, he may be obliged to honor that price.
The key problem here is that the shopkeeper never advertised making wedding cakes for anything other than a marriage between one man and one woman (regardless of sexual preference). But then you knew that already. And no, you do not get to define the shopkeeper's definition of 'marriage'.
More to the point, the anti-discrimination rules that apply to so-called "public accommodations" require that, if you advertise a product or service to the general public, them you must offer it to all customers without regard to race, gender and (as is relevant here) sexual orientation.
My, how far we have now strayed from the Constitution. It's quite comical to see you argue how Colorado law trumps the Constitution when you argue the exact opposite when it comes to Roe.
btw, there was no discrimination based on sexual orientation, as the original Colorado court transcript clearly shows. But then you knew that already.
This is where Mr. Phillips appears to be making his stand, and where I predict he will be found to have violated Colorado's anti-discrimination laws. He can't pick and choose which customers he will sell wedding cakes to, at least not on the basis of the mere sexual orientation of his customer.
At no time did Phillips ever discriminate against any customer based on sexual orientation. The court record clearly shows that he refused the same-gender wedding cake request of both heterosexual and homosexual customers. But then you knew that already before you posted something so blatantly untrue.