Whatever 'principles' these homophobic bakers are hiding behind are based in ignorance and fear. They are not 'principles' but mere excuses to perpetuate more petty repression against a class of American citizens who are committing no crime by simply being who they are.
It doesn't make any difference what their motives are. You have no right to be served by any business, period.
That claim was dealt with decades ago- the reality is you have no right to discriminate against customers based upon their race, religion, country of origin- and in several states- sexual orientation.