> Why do you think so many women occasion a "girls' night out", or men "hang out with the guys"?

Cultural momentum. They inherit the habit from older generations. May as well ask why Swedes love saunas, or why asian cuisine uses chopsticks.

My friend crowds have not had anything resembling those segregated nights out in forever, and in fact, headway is being made insofar women have men in their bridal parties and vice-versa.

> These sorts of things have basically no relevance or importance to social relationships.

You haven't made a case for why the physiological differences between men and women have relevance.