A decree of nullity of marriage in case of voidable marriage, annuls the marriage from the:

A voidable marriage is valid until annulled, but once a decree of nullity is passed under Section 12 it annuls the marriage from the "date of the marriage", not from the "date of the decree". That is why Section 16(2) has to deem legitimate a child conceived before the decree, treating him as if the marriage had been dissolved rather than annulled.