Craigslist doesn't take down "Seeking roommate: female ONLY" because those ads are not illegal.
https://www.craigslist.org/about/FHA
>Federal Fair Housing laws prohibit discriminatory advertising in all housing, regardless of how large or small the property. However, as discussed below, advertising which expresses a preference based upon sex is allowed in shared living situations where tenants will share a bathroom, kitchen, or other common area.
>Shared Housing Exemption -- If you are advertising a shared housing unit, in which tenants will be sharing a bathroom, kitchen, or other common area, you may express a preference based upon sex only.
I HAVE seen Craigslist take down apartment ads that list religious preferences.