It's not about who he slept with. It's about who he was, excuse me, in bed with throughout his tenure at the CIA. Robert Wright has the answer (h/t Kevin Drum)
When, in the fall of 2011, David Petraeus moved from commanding the Afghanistan war effort to commanding the CIA, it was a disturbingly natural transition. I say "natural" because the CIA conducts drone strikes in the Afghanistan-Pakistan region and is involved in other military operations there, so Petraeus, in his new role, was continuing to fight the Afghanistan war. I say "disturbingly" because this overlap of Pentagon and CIA missions is the result of a creeping militarization of the CIA that may be undermining America's national security.
This trend was clear during the Bush administration, but it accelerated under President Obama, who greatly expanded drone strikes, and it reached a kind of symbolic culmination when Obama nominated this four-star general to run things at Langley. That would have been the perfect time to reflect on the wisdom of the convergence of the CIA's and Pentagon's jobs. But, instead, the network of journalists, think tankers, public officials and others who constitute the foreign policy establishment preserved their nearly unblemished record of not focusing on the biggest questions. ...Wright, The Atlantic
The militarization of the CIA effectively screws America because it leaves us without a security service that stays in the business of assessing threats and the actions taken to counter them.
... If the CIA is psychologically invested in a particular form of warfare--and derives part of its budget from that kind of warfare--can it be trusted to impartially assess the consequences, both positive and negative, direct and indirect? ...There have been other military men who headed the CIA, but never has there been one whose move to Langley brought so much continuity with what he was doing before he went there. ...Wright, The Atlantic
Really, it's just another instance of the growing militarization of the US, something we've been warned about over and over again.