Spiritually, why make trouble for a baker?
Then go to a place which sells little plastic brides and grooms, buy two of whichever gender you want, and at the party, stick 'em on there.
(It would be FUN to discover whether or not the goofy folks at Hobby Lobby have a RULE that nobody can buy two cake toppers of the same gender! They're some sort of Fundies, or so the rumours go).
Somebody wants a cake, they show the baker the design they want. No names, no little people.
Baker may have a religious reason NOT to bake a cake for people offending the baker's religion, but, since the baker has no clue that it IS for a wedding of a same sex couple or an interracial couple, or a rich old man and an eager young gold-digger, it's just another cake.
Then, you take the cake to the party site, add whatever decorations you want.
It is dangerous to be that self-righteously arrogant about what one reads.