Petreus was being blackmailed into silence. The question I have is who set him up with the woman? Do you think that this was a simple inner-office affair or did someone dangle an attractive beauty in front of an old man and wait for him to take the bait? Petreus is far from the first older man to fall in the face of temptation, but someone who wanted leverage over him could have easily set him up.
They knew he could be set up because they knew he was weak. The first temptation he gave in to was to accept a position in Obama's administration.