CY- it is a complicated question. I’m not an employment lawyer. Discrimination on the basis of gender and religion is unlawful under title vii, which applies to all employers of 15 or more people.
Note: failing to accommodate a religious need is not discrimination, and has separate rules regarding how hard it is for the employer to accommodate. Thus your post regarding erev shabbos is incorrect.
In my brief googling of the topic I did come across some interesting eeoc guidance re: when religious accommodation can cause gender discrimination, involving a Muslim who wouldn’t shake hands with women. See Nov. 20, 2009 eeoc letter via Google.