In Europe, the railroad cars of the very long International trains are segregated according to final destination. Someone entering in Milan and going to Amsterdam cannot decide to sit in the wagon going to Paris.
Similarly, a gentleman in Costco cannot insist on using the Ladies Only facilities. Segregation?
US Customs also seperates visitors and American citizens upon entry to the US. Isn’t that segregation?
Shoprite has a ’15 item or less’ check-out counter. Is that called segregation?
And the list goes on.
A similar argument can perhaps be made in the Williamsburgh Bus case. Seperation between men and women is optional, expected and adhered to.