First of all it is the priest who absolves us from mortal sin by the power he receives from God at his ordination and Sacramental Confession is most certainly necessary. This is the teaching of the Church which is binding on all Catholics. Why else do you think Our Lord would have instituted the Sacrament of Penance? 
Catholics who are conversant with what the Church teaches will know when they are in a state of mortal sin and that to receive Communion in this state is a further mortal sin. 
A priest would be enforcing the law by refusing Communion to couples openly living in adulterous relationships for the sake of not only their souls but also for the sake of his own. A priest who gives Communion to couples in adulterous relationships collides in the commission of a mortal sin. No bishop can order his priests to commit a sin.