So I didn’t have a source, just recollection. I went to look for a source specifically as it pertains to transfolk and bathrooms.

I don’t know that it’s an easy read, but I thought I’d link to something on congress.gov instead of a website whose bona fides I don’t know.

Although legislation may not alter the substantive meaning of the Equal Protection Clause as interpreted by the courts, Congress may define prohibited discrimination in various contexts, such as in employment and in federally funded education programs. The meaning of sex discrimination in those contexts has also been addressed by federal courts, including in claims brought by transgender individuals. Congress possesses substantial authority to alter the scope of prohibited conduct under civil rights statutes, such as Title VII of the Civil Rights Act of 1964 and Title IX of the Education Amendments of 1972. Likewise, Congress has authority to provide exceptions to the application of those laws, such as the religious exception under Title IX

Harvard Law Review has this to say:

As novel iterations of laws targeting queer identity make their way through state legislatures, an alternative constitutional avenue for challenging them would be to identify and apply the factors that allowed the Romer Court to infer animus and flip the presumption of rationality to strike down a class-based law without applying a heightened form of scrutiny.

I’ll be honest, I’m not familiar enough with laws to fully comprehend what I’m reading here.

Also, I was specifically thinking about Bills of Attainder, which punishes an individual or group without judicial process. One might argue this person is being punished for being trans, but I couldn’t find anything specifically invoking the protection against these in the case of transfolk and bathrooms.