I hate to do this, but here's the whole quote from the Catechism: "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. 'Structures of sin' are the expression and effect of personal sins. They lead their victims to do evil in their turn. In an analogous sense, they constitute a 'social sin.'"

I am well aware that "structural sin" usually means the structures that perpetuate racism. I was asking if Chaput could be using the same logic of "structural sin" to criticize same-sex marriage.