I dont think most Americans 'disagree with' being homosexual
Most of us just find it grotesque and dont want to be forced by law to endorse it.
Just as a photographic design artist should not be compelled to develop and design pornography, this baker should not be compelled to engage in an activity personally that he objects to.
He is on the record saying he would serve these people in other ways, bake them cupcakes, whatever, but not put a message on anything that would appear to endorse what he finds to be morally objectionable.