I guess I'd draw a bit of a line between, say, a generically bespoke item like a suit that would be identical if the dude were marrying a woman as if he were marrying a man.
A cake that says, "Get ready for marital buttsegks!" would require a different level of expression or participation than just making a fancy cake that is made using the color palate chosen by two dudes instead of a man and woman. If the regular passerby couldn't tell the difference, and the point is to make a pretty cake to be eaten, rather than a true artwork, I'm not sure I am all that on board with calling it meaningful participation as expression, rather than simply freedom of association. If you put a rainbow or other symbol of gay pride, then that's a higher level of involvement, and would more closely match the 'would you make a swastika cake' question. So if it's free-expression related, I suppose that I'm more comfortable with content-based allowances rather than customer-based allowances.
A photographer who has to be there and actively participate in the ceremony (assuming they take photos during the ceremony) has a higher level of involvement.
So I'm not comfortable from exempting all services from public accommodation laws, if we are to have such laws, I don't think there's a great deal of value in making, say, mechanics exempt.
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