An american girl in paris (part deux) - S6-E20
Corrected entry: Smith and Samantha discuss how it's been a long time since they've had sex and how it's her chemo that's killing her sex drive. Only a pair of episodes earlier, while she was on chemo, they not only had sex but taped it.
Correction: The episodes don't run as one week, the time scale is indeterminate. Samantha is much further on in her chemo in this episode than she was in the episode where they taped themselves having sex.
Kara