Lotharbot suggested the following standard, which I think makes good sense: if a business provides a generic product, they should not be able to discriminate in who they sell it to, and in turn, we as society recognize that they are not saying anything about support or disagreement with their customers' views by selling them things. If, on the other hand, a product involves customization and expression, the business can refuse customers for ideological reasons, and we can infer from their work what they support.

So a bakery making generic wedding cakes must provide them for everyone, and we as society are crystal clear that this does not imply the baker supports interracial marriage. However, a bakery providing custom cakes based on the couple cannot be compelled to write "Arranged marriage between children is beautiful" on a cake.

A web host is required to sell you webspace regardless of your content, unless it is actually illegal or contrary to technical and ideologically neutral terms of service. But a web design firm may decline to design a page for you based on its content.

I think this is a really good principle, and a great way to preserve both free speech and freedom of conscience.