I've been to a lot of wedding ceremonies. Not one had a cake at it. The cake was always at the Reception, which is not the same as the ceremony.]Baking a cake, renting an apartment approving employment applications and granting loans are NOT moral decisions. Participation in a ceremony, if only so far as to provide a cake IS, in the case of gay "weddings".
It is irrelevant what YOU feel. The baker should not be compelled to violate his religious tenets.
The Reception is the celebration of the union of which they are against.