That is exactly the flawed reasoning rejected by the Colorado Civil Rights Commission.  You're describing a distinction without meaning, because refusing to design for the "event" of a gay wedding is the same thing as refusing to make wedding cakes for gay people.

You should read the decision in that case, as well as the three others where activists attempted to buy cakes from other bakeries with anti gay messages.  Those bakeries offered to sell the activists whatever cake they wanted, but refused to design the requested message.  THAT is the appropriate remedy for the "Christian" baker.