The seller should have the right to sell to anyone they want to, and not to sell. It doesn't need to have a Religious argument to it.
Where in the constitution does it say free enterprise is controlled by the government?
The 14th amendment has been misused and misjudged by the SCOTUS. There was never any real Equality in the Constitution and the amendment was specifically to protect the slaves that were freed.
The 14th amendment didn't give women the right to vote, nor did it give Black Men the right to vote. Both of these rights came from separate amendments, that being the 19th and 15th amendment respectively.
These additional amendments wouldn't have been necessary if the 14th amendment had the power to give equality to the constitutional right to vote.
How does the Income Tax law violate the equality of being taxed the same as everyone else. The tax code penalizes the rich with different and higher marginal tax rates.
As for gay marriage. There is no constitutional right for anyone to get married. The 10th amendment gave that right to the states, then the federal government usurps that right asserting the supremacy clause power. But not directly, it is effectively asserted by the bad SCOTUS decisions and especially those with a 5to4 decision.
For decisions that become the law of the land a majority vote is inconsistent with the significance that decision carries. Remember that the founders only created the legal branch in their checks and balances. But foolishly they gave the job of creating its details to congress. So the constitution doesn't really have much to say about that third branch.
My point is that things found in the history of the US, like the phrase, All Men Are Created Equal is simply not true. And most likely, the word Men is not synonymous with Mankind. As the examples that I already mentioned Women were not given the right to vote.
The rest of these statements like Inalienable Rights are ambiguous and vague and therefore cannot be tied to the rights in the constitution.
Marriage is one of those rights that cannot be tied to constitutional rights. Just because the SCOTUS decided on it doesn't mean it solves the underlying problem. Case in point, Roe v Wade was decided over 40 years ago and its issues are still in hot dispute.
To the point of your question.
I would have the cake seller assert their right under the 13th amendment. Forcing someone to do that is involuntary servitude.
Not all rights are real or valid!