They met when she was looking for Chloe after she (Chloe) faked her own death. She found him in a cornfield after he came down with a bolt of lightning. She was originally planning to meet him anyway. She took him to the hospital and so on...
Also, it happened first episode of Season 4.
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There hasn't been an episode, cutaway or flashback that shows where Peter & Lois were when they got engaged.
Stewie Loves Lois
Season 5|Episode 1 Stewie Loves Lois
D-a-m-m-i-t Janet
I think after that episode of family guy buisness guy, peter gets amneisa and Lois tries to help him remember who he is, but when Lois see him flirting with a another woman she takes of with the kids. and this is quagmire's time to be with Lois
The episode Deep Throats, it is Season 4, Episode 23.