In Act 3, Scene 18, Fainall tells Marwood that cheating on his wife is honorable because it takes place within marriage, an "honourable" institution. Fainall's logic here is circular, a fallacy:
[M]arriage is honourable as you say; and if so, wherefore should cuckoldom be a discredit, being derived from so honourable a root?
Fainall starts from the premise that, as Marwood has said, marriage is honorable. He deliberately takes this as a totalizing statement that can never be refuted, no matter what happens in a marriage. Of course, in a world where marriage is increasingly used to move property around and not just to validate a relationship in the eyes of the church, there are many ways to be legally married without honoring all aspects of marriage. But if all marriages are the same and all behavior within a marriage stems from "so honourable a root," getting married absolves people from any responsibility for their bahavior. Fainall can technically remain "honorably" married while cheating on his wife simply because he is married.
Fainall's use of a fallacy is easy to spot and makes him the object of the audience's criticism. Even though his logic technically holds if the initial premise is true, obviously there must be something wrong with that premise if Fainall uses it to come to this conclusion. By leading the audience to criticize this sham use of marriage as a front for bad behavior, Congreve primes them to question other abuses of "manners." For instance, characters are constantly veiling insults in politeness. Fainall's abuse of logic begs the question of why it is so important to be polite, and whether politeness is honorable or whether it is simply bad behavior cloaked in the image of honor.
