Joe is right, evey self employed person is free to refuse to work for someone, what they cannot do in this case is give an honest reason. both sides are silly on this one ,the gay couple should have just gone somewhere else to get their cake made, and the cake company should have not made a big issue about their opposition to making the cake.
many people are prejudice against black people that is legal providing they do not voice their opinions in public, the cake companies mistake was to make their views known.
I am in favour of gay people having equal rights,but i do not think this case will change anything very much, we already know that certain christian groups have anti gay prejudices and will continue to do so, this cake company will probably still refuse to make cakes for gay people but just not give a genuine reason, they do not have to give any reason.