Season 8's "Jerome is the New Black" revealed the reason why Quagmire is a sex maniac: His jealousy and possessiveness drove away the one woman he truly loved — Cheryl Tiegs — and now he aimlessly has sex to fill the void she left behind. It's actually not all that surprising when you look back on three previous episodes: More of a dangerous pervert due to his questionable methods of seducing women though he refuses to have sex with Meg until she turns 18, and immediately went flaccid when he tried to have sex with Lois while Peter had amnesia and ironically was becoming more like Quagmire as a result, and Lois said she trusted Quagmire as seen on "Big Man on Hippocampus". It's been toned down though, as the show staff have opted to make him more of a Jerk with a Heart of Gold.
Family Guy: OC Universe Chapter Amazon Resort, a family guy fanfic | FanFiction
Family Guy - 08x13 - Go, Stewie, Go!. But where are those good old-fashioned values on which we used to rely? Lucky there's a family guy. Lucky there's a man who positively can do all the things that make us laugh and cry. Inch-and-a-half left of your part. Oh, my God, you're right!