He didn't refuse to sell them a cake. He refused to utilize his artistic talents to promote an agenda which he did not support.
This is a first amendment freedom of expression case. An artist should be free to pick the subjects in which he will exercise his artistic talents.
IIRC, he actually told them that he'd make them any other kind of cake. So, he didn't refuse them a cake.
And how is this different than a muslim caterer refusing to prepare ham for a wedding? We know they'd treat it differently. Even these gay customers would have treated it differently...they wouldn't have asked in the first place if they knew he was Muslim, or if they found out he was a muslim they'd change to beef and lamb and think nothing of it.
Why? They'd say it was tolerance for differences.
But there's no tolerance for Christians. That's why Obama bullies nuns, why Saudi beheads them, and why gays take them to court.
Christianity isn't permitted to be itself. Everyone else is, though.