And who says the 1st amendment is being violated by applying general laws to Christians?
You do. Citing you. And much like your insistence that you alone define what marriage is, there's no need for an amendment because you disagree. Contrary to your personal beliefs, Christianity doesn't trump all civil law.
See how that works?
A custom decorated cake is an artistic expression. The First Amendment quite directly protects an artists right to make such an expression,and equally to refuse to make such an expression if he does not agree with the expression that someone wants him to make.
Using the force of law to compel an artist to create an expression in support of a celebration of something that is evil, and in which that artist wants no part, is a direct violation of that artist's First Amendment rights.
The same would be equally true if we were talking about an artist that did not want to create a work in support of a Ku Klux Klan event, of a Westboro Baptist Church
“God Hates Fags!” event, or anything else that violated his own moral and ethical values. Your right to hold a particular belief, to express this belief, and to celebrate this belief, does not include a right to force someone else who disagrees to participate in an expression or celebration of this belief.