“Nor are appellants penalized for refusing to create wedding-related merchandise as long as they equally refuse similar services to opposite-sex couples.” So, they aren't penalized for refusing to provide custom artwork for same-sex marriages, provided they do not provide custom artwork for any marriages. Such astounding logic!
I'm not sure, but in context I think it means that they might have moral objections to some things opposite-sex couples might want to have cards designed for. Maybe their impending divorce?
I'd venture that they wouldn't design products for Satanist weddings, regardless of the "couple's" sexual orientation.