It is perfectly legal to ask, but that opens you up to accusations that you also discriminated. You no longer have the defense that you didn't know the informat

It is perfectly legal to ask, but that opens you up to accusations that you also discriminated. You no longer have the defense that you didn't know the information, and the court would want an explanation for why you were asking. It's hard to imagine an illegal question under US federal law, but I suppose there could be one where the question itself is sexual harassment or an attempt at stealing trade secrets.