The more critical answer is that religion has a special place in the US, and is among the foremost of rights. A business which develops religious art for any of their customers except with regards to Christian art would also be violating constitutional rights, despite the fact that their non-Christian offerings are available to all.

Also, we could pick easier discussions like how religious organizations have broad freedoms to discriminate against gay individuals, such as by firing gay employees or evicting gay students. There it becomes clear that religious rights have a very special place in law, and that gay marital benefits by the likes of Microsoft or Google is simply an expression of soft company power.

Furthermore, gay protections exist at the federal level largely due to a previous Supreme Court with a Sandra Day O'Connor acting as a pivotal vote. It is not obvious how Lawrence v Texas, which legalized gay sex, would've turned out in today's court.