If the homosexual couple disagreed with the printing on the boxes, well thats just too bad.
How is that different from the homosexual couple knowing that the baker doesn't want to make the cake for them in the first place? Your solution doesn't solve the problem.
The issue is how NOT to sell to people who want you to make something that violates your faith, given the recent court rulings and laws in this area.
What I propose solves that problem.
It solves the problem in that no other homosexual couple will ask them to bake them a cake.