It's hard to think of anyone not getting it in the neck from a biographer certain that he knows what it is to live decently. And Koestler, some of the time, did not live "decently." Of course, what he did most—and most interestingly—was write. But when he wasn't writing he often drank, sometimes heavily, he drove his car when over the limit, he got into arguments and fights—and sometimes all of the above on the same night. And then of course there is the sex. Koestler was very keen on it (he even co-wrote a couple of encyclopedias on the subject); he didn't believe in sexual fidelity, either from a biological or temperamental point of view; he found, when in a prolonged sexual relationship with a woman, that an "incest barrier" erected itself (Cesarani takes this to mean that he felt he was sleeping with his mother, but surely the implication is one of sisterly familiarity); he chased, with success, a large number of women throughout his adult life. 

At times, the habit became compulsive to the point where Koestler was dismayed, even sickened, by his own hedonism; but Cesarani has got there long before him. When Koestler is twenty-six and still unmarried, he is rebuked for "lust and perpetual infidelity," and for "sleeping his way through Berlin at the rate of one girlfriend every four to six weeks" (it's to be hoped Cesarani is never signed to write the biography of a rock star). A continuing misdemeanor of Koestler's was to remain on good terms with many of those he had slept with; so at one point he has a regular "sociable lunch" with—and here Cesarani's adjective should be exactly weighed—"a grisly assembly of ex-lovers." When a "strikingly pretty young Turkish woman" who works for the UN writes to Koestler offering to "rush at" him, and subsequently comes to London and effects the offer, you might judge this a free and as far as we know painless exchange of sexual favors between consenting adults. Cesarani takes it as proof that Koestler's "sexual opportunism" went to "astonishing lengths."(Whose opportunism?)