Sorry, yes, if it is a private sale, you can sell or not to whomever you please.
A business, however, generally has a license issued by a local governing authority that is tasked with enforcing the law, including those against discrimination. That permit can be revoked if the business does not conform to those laws.
I could care less whether you agree with the law. If you don't, lobby to change it. In the meantime, you have two choices.
You can obey the law, keep your permit, and keep doing business.
You can ignore the law, and risk paying the penalties.
Look on the second option as an opportunity to become a martyr for the cause; to defy the law, spend everything you own fighting it, possibly end up in jail for contempt of the law, and so on. Then when you die and stand before your god you can beat your breast and tell him how dedicated you were to the cause. Might get you a mention in some heavenly journal and a house next door to Job or Daniel, or at the least a pat on the head and a cookie.
No one is saying you can't ignore the law.