I haven’t seen the movie yet. Maybe a character is (or was) a lady’s man?
In any case, considering that all women are presented as cheating wives, the title seems to me mainly a curious variation on the S.O.B. theme typical of the early '70s*.
The part before the suspension point could also mean “All comrades in the West”, but what follows makes it abundantly clear the meaning is literal.
Indio Black, sai che ti dico: sei un gran figlio di… (September '70)
Monta in sella, figlio di…! (February '72)
Trinita e Sartana figli di… (May '72)
E continuavano a chiamarlo figlio di… (June '72)
Alleluja e Sartana figli di… Dio (December '72)