I agree, but when its a specialty created item - that's completely different, then you know what their behavior is and are participating in its celebration. Wedding cakes arent blank and sitting in the case to sell to people, its a specifically created item.
No, you're mixing eggs and flour. If you were there at the reception, that's celebrating. Doing your job ain't the same thing.
For that matter, supposing a gay couple doing a quick city hall marriage grabbed a cake off the shelf and mentioned "Hey, it's for our wedding, we're eloping," what then?
P.S. Are you saying craftmanship always implies "celebration" of the event or end user? What if a guy making an ice sculpture for a 50th anniversary for the Genoveses hates Italians? Is he "celebrating" their anniversary or just doing his job? What if a photographer does a shoot for a magazine and hates the subject of the cover? Is the photographer "celebrating" the subject, or just doing what he's good at? If a jeweler crafts a specialty work for a Muslim couple and detests Muslims, is she "celebrating" her customer's religion, or merely doing good work?
And so forth.
If anything, a real mercenary case could be made that taking the money from these people and putting it into your pocket is at least an economic victory for you that could be put to good use.