Yes, this is exactly right.  The issue was never about imagery on the cake itself, it was what the cake represented, and the fact the Baker did not agree with gay marriage - although, he seemingly had no issues baking wedding cakes for people who committed adultery, or had been divorced, or even if the person had stolen things in the past, or even murdered anyone.  Apparently, for this baker, the definitive factor on if he will get into heaven or not is if he ever baked a cake that was later on used in a gay wedding.

And I agree - if the couple had wanted some image on the cake that the baker found offensive, or was not a normal image/item that he had used in the past for other wedding cakes, and the baker refused on those grounds, I would be much more inclined to side with the baker.  But that is not the case here.