I have seen this comment several time on FR, and in truth it baffles me. Is it based upon the assumption that the Church's teachings are so burdensome that any given week a sizeable minority of the parish would have to abstain from communion? I would assume the exact opposite, the better the catechisis the less likely for the need for persons to abstain from communion, due to better comprehension of the Church's teachings. In addition, better cathecisis would result, I would assume, in greater respect for the gift of confession, and then again, the need to abstain from communnion would not exsist.
True there are two ways to look at this. At the local liberal church masses here, I often see the scandal of 100% of the people going up for communion, because they've been told (or assume) their parish "does confession" once a year at Easter. But we're fallen creatures and most all of us fall into mortal sin quite regularily as anyone you reads a good guide to examination of conscious can admit. Thus in reality, most who do the Easter confession ritual and ignore confession for the remainder of year commit mortal sin every Sunday morning by receiving the body of Christ unworthily. (A similar we-are-the-church mindset like the the happy couple above).
I believe (or would hope to believe) that the smattering of "left behind" people not going up for communion are not ignorant of the sacrament. Rather they're a mixture of those not yet prepared for confession that week (examinations or prayers etc), and and those you KNOW they're in a situation (like co-habitation ) which presents makes them not worthy to receive.
The "left behinds" know it is more pleasing to God to not receive unworthily, and seek absolution as soon as possible if one is in a state of mortal sin.