Section VII of the Civil Rights Act of 1964 clearly states that no person should suffer discrimination with respect to compensation, terms, conditions, or privileges because of their ethnicity, colour, sex, religion or nationality. It specifically lists restricting the use of company facilities as a form of discrimination.
The Civil Rights Act clearly demonstrated that forbidding a transgender individual from using the restroom that corresponds to their expressed gender would be a form of discrimination. They have as much right to use the bathroom of their expressed gender as a cis man or woman does. The trans community are protected from discrimination in this way by the clause in the Civil Rights Act which refers to sex-based expectations. The Obama administration shared this view and protected the trans community under federal anti-discrimination laws.  Even if it is not already enshrined in law, it should be. The First Amendment protects citizens' religious freedoms. This is essentially a choice about a person's personal identity. The transgender community should be protected by the same amendment. Those individuals have made a choice about their personal identity in the same way and should receive the same protections that come with it.
Transgender citizens are not automatically covered by the Civil Rights Act. It refers to sex but not gender. It is up to the individual state to clarify what segments of the population are protected by anti-discrimination laws. Currently, most states don’t include the LGBTQ+ community in anti-discrimination laws. Only 20 states specify that citizens can be discriminated against based on gender identity. The rest are free to force trans citizens to use the restroom which corresponds to their birth sex.
In the United States, discrimination is illegal. Restricting restroom access based on gender is a form of discrimination.
Anti-discrimination laws do not apply to discrimination on the basis of LGBTQ+.