It's interesting to think back to all those cake bakery scandals. They look a little different in hindsight.
I think that a cake maker shouldn't be allowed to refuse to sell a cake on the grounds that they don't like the sex, colour, or orientation of the buyer.
However, they should be allowed to refuse to ice the cake with a political or religious message that they disagree with (be it pro-gay-rights, anti-gay-rights, pro-TRA, pro-GC, pro-BLM, anti-BLM).
So then the grey area in my mind is whether they should be allowed to refuse to ice it with "congratulations on your wedding, James and Michael" - does it fall more into the first category or the second one?