So is asking someone to write "oppose gay marriage" on a cake hate speech then? Also I certainly don't remember the left defending the gay baker's refusal on the grounds of turning away all political speech on cakes either, they defended his right to refuse this particular political speech. How about speech that's just in questionable taste, like "Hitler did nothing wrong"? Or calling Mohammed a pedophile, or any other number of things a business would find objectionable?
Basically the left wants their cake and to eat it too. They're fine with limiting speech from people they dislike (Westboro Baptist Church, anyone opposed to gay marriage, any conservative trying to speak at a public college, etc) and use their heckler's veto to shout them down. But they'll try to crucify any misguided soul who expresses anything less than an enthusiastic embrace of their pet minority groups. I'm not advocating for the right for those people to be discriminatory towards gays or anyone else, I'd just like the left to be intellectually consistent and they aren't.