Do you believe a business should be able to refuse a wedding cake to an interracial couple? Because bigots find race-mixing offensive and against their "beliefs".
Staples prints church materials, it doesn't make them a Christian business. I painted a mural for a church in VA. I'm an atheist. If you're a baker, baking a cake doesn't endorse anything. Do you think your bakery gives a shit about your birthday? They're not pro-(insert your name here), dumbass. They just bake cakes.
It's not about the goddamned cake. If you don't want to serve someone you might not agree with then don't go into the fucking service industry. It's really that simple.