You understanding is wrong. Not because customers are gay. The Christian bakers do not against gay people, they just do not want to make cakes for gay wedding. It's a huge difference.

Should a Christian church be forced to host a gay wedding?

Can you force a Muslim restaurant to serve pork meat to you? If a Muslim restaurant does not allow you to bring pork meat into their restaurant, is it a discrimination against you?

If your argument is that religion cannot be allowed in any business decision, you are the one totally lost sense.