Charity does not insist on its own way. In this solidarity with all men, living or dead, which is founded on the communion of saints, the least of our acts done in charity redounds to the profit of all. Every sin harms this communion. Sin is a personal act. Thus sin makes men accomplices of one another and causes concupiscence, violence, and injustice to reign among them.
Sins give rise to social situations and institutions that are contrary to the divine goodness.
They lead their victims to do evil in their turn. In an analogous sense, they constitute a social sin. When we weigh these consequences against the fleeting pleasure of sin we can see why the Church teaches that sin is an act contrary to reason.