Supply a cake IS participating in an event when the baker has to design it and most wedding cakes are usually delivered by the baker to the reception hall and assembled with finishing touches there.
Having to provide decorations specifically for an event is participating.
There is no constitutional right to a wedding cake, nor is there any constitutional prohibition to having one. And there sure isn't a constitutional right to force somebody to participate in something he/she feels is wrong. If the baker is out of business, who provides the cake then? Or if a baker isn't willing to take on the job, friends and family of the gay couple can do what I, with the help of others, have done for many friends and family who couldn't afford a professional cake or couldn't find a baker who would accommodate our unusual request--bake and decorate a cake ourselves.
And it still all comes down to the right of any person to choose what practices, events, occasions, etc. it is appropriate for him to be a part of. And that goes for all people, white, black, gay, straight, or name your flavor of choice.