Designing a website may be an artistic expression. But WEBSITE DESIGN does not include deciding what the website will say.
And that is the point. A wedding website is not a gay wedding website until someone adds specific CONTENT. You can design stationary, and that would constitute an artistic expression. But the customer who buys the stationary is the one who puts in the content.
So if a business can refuse to sell a WEBSITE DESIGN to someone because the designer has a religious objection to the content the customer will insert, then someone selling stationary can also do the same thing. And for that matter, a restaurant owner can refuse to sell food to a black person if the owner professes a religious objection to the black person being there. And it goes on, and on, and on.