I think we are too quick to think in terms of rules in both the moral and spiritual life (which are really the same thing), without realizing what those rules really mean. There is a rule, for instance, about going to confession before receiving communion if we are in a state of serious sin, and lots of lists of what sins are "serious". But the point is surely that the spiritual effect of communion depends on the quality of our self-gift. When the priest prepares the offerings at the altar, we are attempting to join ourselves to those gifts, so that the self we give to God in them can given back to us transformed. In receiving communion we receive into ourselves not just the host, but Christ himself, and in him the new self that he is wanting us to become. But sin is always a form of attachment to the old self. Thus if we are in a state of sin before Mass we are in a state of attachment, of holding back part of ourselves from the gift - and this part of ourselves cannot be transformed. Going to confession is a way - the best way, sometimes the only way - of detaching ourselves from ourselves, so as to benefit from the Mass.