If it is a business open to the public, then you cannot deny service based on race, sex, country of national origin, religion or sexual orientation. That, by definition, is discrimination. 

Why is this difficult to understand? There are no exceptions.

If the farmer denies a LGBT couple to rent his property for their wedding while allowing others then he can be sued or fined.

If the farmer will not sell fruit to LGBT customers then he should be closed down.