And I'm sure people would understand if a request was made that went against a Muslim beliefs. For example, if a bakery was
Nope, I'm going by what you said the standard for refusing service is. You said it's ok for blacks, jews to refuse kkk, nazi cakes because it demeaning, insulting to a certain group. I'm saying if that's the standard, then that applies to a certain group as well. Which in this case is religious. (Technically, you can say Jews is for religious reasons as well. Depending on the Jew and if they practice their religion). As far as, saying that therefore all things not biblically prescribed is an insult. That's an assumption. I never made that argument. Sure, there's never been a movement of gays doing those things to Christians. Still doesn't mean it's not an insult and demeaning to what a Christian believes God says marriage is.
As far as the last paragraph, no that would not be ok. If religion beliefs should not be encroached upon then that would apply to Christian as well. And that gay religion would be encrouching upon the Christian to make them a gay marriage cake.