Found the quote but I can't remember exactly which episode it's from.
Brian: Remember that time you narrated your own life?
Peter: "I walked into the kitchen and sat down at the table. I looked with a grimace at the questionable meal Lois had placed in front of me. Of course I'd never tell her how disgusted I was with her cooking, but somehow I think she knew. Lois had always been full of energy and life, but lately I had begun to grow more aware of her aging. The bright, exuberant eyes that I had fallen in love with were now beginning to grow dull and listless with the long fatigue of a weary life. [Lois knocks Peter unconscious.] I woke several hours later in a daze."
it's the aformentioned episode 'peter griffin: husband, father...brother?' where peter learns that he has some blackness in his ancestry. the one where brian is overcome with urges to do it doggystyle is 'screwed the pooch'