Why was he (spoiler)

We see them kissing when he visits her to tell her that the boat will be repossessed. And while we don't get to see anything more than kissing, a lot is implied. John is so ashamed when sneaks into his bedroom at 11:30, when his wife is already sleeping. And Mona is so distraught when she finds out that John is married and has a child, so we have reason to believe that they had been intimate with each other by that time. Also, when John gets Mona's telegram, he looks quite worried. What kind of urgent news might she have? Could she be pregnant? Actually, it's about Mac, but before they part, he asks her "Everything else okay?", as if to make sure that she is not hiding the consequences of their affair from him.
Later, Mona admits to Smiley that Mac does have a reason to be jealous of Forbes, but that it's all over. Smiley (rightfully) says that for something to be over it has to start first, and Mona does not protest. She did have an affair with John.


You may cross-examine.

JohninVegas, either you didn't watch the movie very closely, or you don't know much about pre-sixties cinema...

The "making whoopie" was implied.
If it wasn't implied I imagined it. So, there.