Why would you hate to clarify for me? 

With the full quote in hand, I think I might now understand how you (and the Catechism in that section) understand the concept of structural/social sin. And if I am right, then your usage seems to me to be different than that used by the author.

Do I have this right?

In Chaput's mind (and yours?) every instance of homosexual intercourse is a mortal sin because it is contrary the natural law. Gay marriage is a social structure that arises from individual instances of the mortal sin of homosexual intercourse, and in this case the very purpose of the social structure is to facilitate more sin of the same type. Therefore, "in an analogous sense, [gay marriage] is a 'social sin.'"  Thus, the only real sin is "personal sin," the only sin for which someone must answer is individual personal acts; gay marriage is merely analogously sin, sort of sin like. 

If so, then do you realize the authors use sees social sin differently? 

Thus my confusion.