True (or rather, they don't want to normalise the idea that men can have husbands). But the operative word here is "want."

Again, to lean on the segregation example, I'm quite sure many business owners didn't want to normalise the sight of a black person eating or shopping or going to school with a white person. But they weren't given a choice. The law compelled them to offer their services equally, because anything less would be discrimination on the protected characteristic of "race."

Gay marriage is recognised (normalised if you will), by the state. Gay men, therefore, can have husbands. So a private individual or business doesn't have the right to discriminate on that basis. If you can think of a message for a cake that I'd be offended by that's neither discriminatory or a matter of opinion, I'd be interested to hear it. But otherwise you're not comparing like with like.