This all sounds like the staff using religion as an excuse to discriminate against gay people. Doesn’t seem all that Christian to me, and in fact it seems like they’re taking Our Lord’s Name in vain by using it to justify their hateful actions.
But maybe they don’t follow the teachings of Jesus Christ and don’t follow the Commandments. Even if that’s the case, the business is responsible for ensuring that customers aren’t discriminated against by staff. If the business owners aren’t up to meeting that standard, then they shouldn’t be trying to run a business.