What do you think is Congreve's purpose in establishing an affair between Fainall and Mrs. Marwood?

Congreve possibly chose the affair between Fainall and Mrs. Marwood to show the scheming, deceit and lies common in Restoration society. He is able to do this through general characters and doesn't have to lampoon anyone. In doing so, he has made a timeless play which we can still enjoy today.