Saifer Hasidim 1120 brings that you can not say shehasimha bmoinoi if woman are sitting with men
So it seems to me that halohacly it is not required, but it is preferable
As @sam commented below this minhag (not to say shehasimha bmoinoi) is brought in the BaCh (on the tur)
Sam's comment
"http://beta.hebrewbooks.org/tursa.aspx?a=eh_x7291 see the Bach (12) from the words B'krakow"