A portrait painter who works on commission is not a public accommodation subject to the anti-discrimination laws at issue here. But even if he were, he could refuse to paint a portrait deemed offensive. Just as the baker can refuse to write "God loves gays" on a cake. But the right demanded by these bakers appears to be to turn down the business ab initio. Not on the basis of the customization required or the particular message sought for the cake. But simply because the cake will be eaten at a reception for two gays who've chosen to marry under the civil law.
OK, now here's a good example of a dishonest argument. If the couple had asked for a plain cake, I expect that the baker would have provided it. The problem is that they wanted a same-sex-wedding-cake, as in with two men or two women as the topper and some sentiment expressed about their nuptials. That's a problem for a standard-issue Christian.
"simply because the cake will be eaten at a reception for two gays who've chosen to marry under the civil law..." Jazzhead doesn't know this to be true, however he makes the claim that it is. That is dishonest.
Additionally, the baker never knows who will be eating his/her cake. Will there be gays at the wedding? Men and women living together but not married? People with criminal pasts? Obnoxious men and mean women? Maybe, but no way to tell nor any reason to. That's a dishonest argument.