What was the storyline in the play Sexual Perversity in Chicago?

The storyline in this David Mamet play is fairly sparse and is actually secondary to other things such as character development.

The play has four character, Dan Shapiro, Bernard Litko, Joan Webber and Deborah Soloman.  The first two are friends, the second two are friends and room mates.

To the extent that there is a plot, it is about Dan and Deborah.  They meet, fall in love, and then eventually fall out of love and come to hate one another.

While this is the storyline the more important parts of the play show the two sets of same sex characters talking to one another.  These scenes provide insight into the relationships between the same sex friends and how those relationships are affected by the Dan-Deborah affair.