I think it is a 13th Amendment issue. If they sell general pre-decorated cakes, then they can be required to sell them to anyone who asks. However if the cake decorator does not wish to decorate a cake with some message they find repulsive, then forcing them by law to decorate that cake is involuntary servitude. Decorating a cake is art. You cannot compel an artist to violate his own conscience to make art that he finds repulsive.
The same goes for florists, photographers, preachers.
Most of these "sexual orientation regulations" are also administered by unelected commissions, without the due process guaranteed by the Constitution of a trial-by-jury option. I'd like to see one of these attacked business owners demand to be tried in a duly authorized court OF LAW in a jury trial, with a high-powered attorney defending him and having a hand in selecting the jury. Even if one juror can't find him guilty, he walks, and the state has to foot the bill for the expenses. Maybe they'll think twice before they decide to prosecute again. It's all about money and guaranteed narrative victory (which a tribunal by commission guarantees) with these fascist goons.