Evidently everyone is referencing the Colorado case, while the prominent
Ireland case was much closer to this scenario, in that the cake requested had a clear political message and image of Bert and Ernie, and the bakery was (indisputably, in my mind) unfairly punished for refusal.
But even the other cases are not so simple. I don't believe we're looking at a scenario where someone walks in and picks up a cake off the shelf, but is told they can't buy it. There's a reason this keeps happening with bakeries and not with stores that sell you a packaged item: it's a commissioned work, it is often seen as a creative act (varies, I'm sure), and it is a kind of embodied celebration of an event that is seen as a religious ceremony to many.
A wedding cake generally follows a meeting with the purchasing parties; anyone who has been directly or indirectly involved in planning weddings knows that you don't just pick a cake in most cases, but instead you go and work with a baker to get what you want for your ceremony. It's much more like a commissioned work of art than buying a food item. And on top of that, it would seem that the Christian-centered bakeries were built partly on the idea of meeting with and serving couples for weddings as something meaningful to them, participating in the celebration. That's my first-hand experience from speaking to a wedding cake maker in our case, that they are being commissioned to create a work to celebrate something, and that this is their identity to a significant extent. Although all bakeries are different in nature, it's not absurd to imagine a Christian bakery feeling trampled on unfairly if coerced into undergoing this same commission process for events that directly contradict their beliefs, and that to their eyes even represent a parody of their religious ceremony.