I'm quite convinced that the baker was probably indeed prejudiced against homosexuals

Why?
No other complaints emerged despite the publicity.
He says he served gay folks.
Why would he not make a cake for one.

It was just the slogan, which he would have refused whoever asked.
No discrimination.

If this decision is allowed to stand, expect different activists to request the same slogan from Muslim bakers, printers etc.
How many such folk are we prepared to see dragged through the courts?

It is not reasonable to demand anyone to produce propaganda material for causes they do not approve of, or to criminalise them for refusing.