The law does not mandate people use the bathroom of their biological sex wherever there is a public restroom. It mandates that when multi stall restrooms are provided by school districts and other government bodies such multi stall restrooms will be provided for each sex. It states that if a place of public accommodation does not provide gender neutral restrooms that shall not be considered a discriminatory practice subject to state oversight and sanction.
A business is free to provide single use restroom (as are the political subdivisions and school districts) provided the plumbing code allows for that number. They can even provide a gender neutral multi stall bathroom. That is their concern not that of NC.
When public agencies and educational facilities provide multi stall occupancy restrooms then those are to be used only by those whose biological sex that is designated by the restroom.
The only place where the state tells a place of public accommodation how many restroom facilities they have to provide and how to designate them is in the plumbing or building codes.
Laws about a person of the opposite sex going into the wrong restroom of a business would be a local matter. If a law was passed prohibiting such conduct it could not be regarded as being discriminatory.
A private business or organization should be allowed to do what it wants, discriminate on any basis. Anything else is tyranny. We have lost the freedom of association and the government is using anti-discrimination laws to run our lives. Only government cannot be allowed to discriminate.