She doesn't, there's no such episode.
There are episodes in which she becomes a Martial Arts expert, Lethal Weapons and an episode where she becomes a boxer, Baby You Knock Me Out
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Your question makes no sense with "go in Lois butt", as there is no such episode where 'Stewie goes into Lois' butt', however you may be thinking of Season Four Episode 26 Petergeistwhen the exit of the spirit world was Meg's rear end.
Big Man on Hippocampus
Lois and clark kiss in the episode "crimson" but Lois is on a drug and clark gets infected w red kryptonite so its not technically real. Also in episode "hydro" but Lois thinks its oliver she is kissing because he is in the green arrow costume. in next weeks episode " crossfire" there are pictures and promos of them kissing as well.Season 8 | Episode 13: "Go Stewie Go"
I can't find an episode where they both are, but you may be recalling S07|E13Stew-Roids when Bonnie asks Lois to put lotion on her back.
The Blind Side