The baker doesn't only make wedding cakes. I'm pretty sure that it is a rare baker who could even make a living only selling wedding cakes. But it does not matter whether the reception is part of or separate from the wedding. If the baker does not wish to be a part of that event he should not have to be any more than he should be required to participate in an event requested by black people if he had moral, philosophical, or ethical problems with it.
Should a gay baker have to design a cake requested by black people for a seminar that featured objections to gay marriage just because the reception would be in a separate location from the lecture hall? I would go to bat for that gay baker in a heartbeat because nobody should have to support in ANY way an event with which he/she has moral, philosophical, or ethical problems. I would not design the cake for those black people either in that circumstance, but I would emphatically defend a black baker in not having to bake a cake for an event at the Westboro Baptist Church or the local KKK convention no matter where the cake was set up and consumed.
In order for personal convictions be respected and tolerance to be real, it has to apply to everybody, even when we ourselves do not share the conviction of another.
Once again, any and all of such people and all people should cheerfully and courteously provide whatever products they normally offer to sale to each and every customer who comes in no matter what their personal attributes or flavors might be. But nobody--NOBODY--should have to participate in an event in ANY respect when he/she has has moral, philosophical, or ethical objections.