Petraeus is a democrat. It doesn’t matter that long ago he was registered as a republican in New Hampshire. He quit voting, and he took a job with the Obama administration. He’s a democrat.
If you believe that a democrat has to resign a government post simply over an affair, then you would believe that the attack on the the U.S. embassy in Benghazi was instigated by a video, which was viewed by about 20 Californians.
Update: FBI investigation — Petraeus is said to have become an obsessive nutcase. If so, he should have been fired a half year ago when the investigation turned this stuff up.