A man having an affair with a married woman discovers his wife is having an affair with her husband.

It sound interesting a comedy about that. You have a plot point but not the protagonist's motivation... what should he do? what must he do? Why did he do that?? You can add an adjective? that answer that, for example; a stressed man or a party man. I hope I helped.

Made me think of the stage play "The Virtuous Burglar" by Dario Fo. It has similar plot points, so you might want to check it out.

Agree with Richiev.

As Woody Allen, who knows a thing or three about comedy, said: "Good ideas are a dime a dozen: 'It's the then what?' that kills you." The logline needs to answer that question. Then what? What is the objective goal the protagonist settles on to deal with the situation?