It's a good example of what the Lefties hypocritically call "cultural appropriation." (The Lefties never acknowledge that anything they THEMSELVES do is "cultural appropriation.")
The gay couple usually take great delight in mimicking the traditions of matrimony, often in an "ironic" or burlesque way, with one of the gay partners getting dressed up in the formal clothing of the opposite sex, etc.
The whole thing is an ostentatiously self-righteous and thus crypto-hateful behavior. And so, of course, the gay people often react with melodramatic shock and outrage when the rest of the world, including cake decorators or photographers or florists or musicians, etc. etc. etc. refuse to get involved in such demonstrations and exhibitions of socio-pathology and hate.