I suspect that the reason that the sixth commandment of the Church was removed was because the other commandments are based on discipline and can be changed while the commandment on marriage is based on divine law and cannot be changed. For example, we have the obligation to assist at mass every Sunday and holy day of obligation. However if I live far away from the church and am unable to get to mass or if I am ill then I am excused from the obligation to attend mass on those days. However one is never excused from obeying teh laws of the Church regarding marriage. Marriage is until death do you part and one is never allowed to have marital intercourse outside of marriage. Doing so is grave matter and would need to be mentioned in confession.