You'll find a good summary of the relevant rules at https://www.craigslist.org/about/FHA.

If it was a roommate request, then it is viewed differently. If the property was at most 3 units and the landlord lives there, then the landlord cannot advertise that they will discriminate on gender but the landlord is allowed to. (Who said that the laws had to make sense?)

Yeah if I'm going to get declined after going to the trouble of applying, or if I'm going to enter into a housing agreement with someone who hates me, I'd rather their advertisement be smothered in discriminatory epithets. Seriously.

But if you're just browsing past the ad, do you need reminding of how many discriminatory slimeballs there are out there?

I don't like the mentality that leads to this kind of confused legal situation. But if I squint sideways and pretend that I don't have a brain, I can kinda see how someone could have thought this a good idea.

I'm curious what the circumstances are. The Barker case was open and shut - not only was it racially motivated but the defendant literally sent a text message incriminating herself.

If your host sent you a text message after canceling saying "I hate men, that's why I'm canceling you" I bet you'd find a lawyer who could take the case.

Circumstances differ, but they clearly state it was because of gender. It was a declined request, not a cancellation, and there no political overtone to the statement. Not sure about other details like the occupancy, or use by the landlord. But they said they didn't want a male, and that was all.

I imagine it would be the same. See "Who is protected?" on this page: https://www.dfeh.ca.gov/Housing/

Sex/gender is included.

It depends on local laws, particularly if the owner lives in the dwelling. There are many caveats and carve-outs.

While it's somewhat understandable for some types of rentals (e.g. a spare bedroom in a house), I would think not unless there's some fine-print exceptions. [ADDED: Apparently there are for smaller shared housing when the landlord lives there.]