If company A is entitled to discriminate against party B because of the things the latter says and/or does -- how and why do you stop business C from discrimina

If company A is entitled to discriminate against party B because of the things the latter says and/or does -- how and why do you stop business C from discriminating against party D because of their sexual orientation. A: hosting company (GoDaddy today) B: anyone (Spencer today) C: bakery D: gay couple I think a Twitter equivalent of this provocative argument is: now do this for restaurants refusing to serve certain people because of the color of their skin.