There is no practical way of enforcing a prohibitive bathroom law
A law dictating what bathrooms the transgender community must use is unenforceable. Who will police the restrooms? Will they check the government-issue ID of anyone suspected of using a bathroom for the sex that isn’t on their birth certificate?
We shouldn’t even be discussing the question because the idea of enforcing such legislation is out of the question. Transgender people will continue to use the restroom of the gender they identify with. In cases where transgender students have been told to stop using a bathroom, they have been reluctant to do so. Short of placing security on bathroom doors to check identification documents (which is not being discussed), trans people will continue to use the bathroom they feel comfortable in. In Pat McCrory’s own office in North Carolina, he wasn’t able to stop a transgender citizen using the bathroom of their identified gender. Mara Keisling, the head of the National Center For Transgender Equality used the women’s restroom while working to repeal North Carolina’s “bathroom bill”.
It is enforceable with public cooperation. If the government legislates to force transgender people to use the bathroom for the sex written on their birth certificate, then it legitimises the public to take policing into their own hands. If they suspect someone of breaking the law, they may ask private and probing questions themselves. If they suspect illegal activity, the member of the public could phone the police. It could make the experience of using a public restroom even more uncomfortable for trans people and lead to an increase in harassment, but if they are breaking the law they must face the consequences.
Transgender people have continued to use the bathroom for their identified gender, even when it is illegal. There is no way of enforcing it without checking IDs on bathroom doors, which is not practical.
It is enforceable with public assistance.