From "Tinker Tailor Soldier Spy: Flushing Out the Mole (#1.7)"
[on his affair with Ann
The thing with Ann was Karla's idea. He always believed that if there was ever any threat, it would come from you. He said you were quite good. But you had this one weakness: Ann. It was a double-fix, actually. On the one hand, you wouldn't be likely to think of me as a Circus traitor if you were preoccupied with wondering what we got up to in bed...