When the baker says that his wedding cakes are a custom work of art tailored to each client, he's bullshitting you. He simply wants to discriminate against gay people. It really is that simple. There is no material difference between drawing flowers on a cake for a gay wedding and drawing flowers on a cake for a straight wedding, the same as there being no material difference between drawing flowers on a cake for an interracial wedding and a non-interracial wedding. When people make up false pretenses to justify their discrimination, your job as a rational human being is to identify it as such, not be a sucker.
I absolutely guarantee that if a straight couple had came in, commissioned a regular wedding cake, and then once the cake was done said "Oops we're not gonna buy it, can you sell it to our gay friends' wedding instead?", the baker would not have sold them the cake at that point, because again, it's about who he's selling it to, not what he's making.