Although "The Big Bang Theory" is relatively mild and family-friendly, at least compared to its contemporaries like "How I Met Your Mother" or "Modern Family," it did still regularly delve into sexual humor. Howard's always got some skeevy joke to tell and there are no shortage of jokes about Penny's sex life. With Sheldon (Jim Parsons), however, the show was borderline G-rated; the character had seemingly zero interest in sex for the first few seasons, and even by the end of the show, the evidence of his libido was still scant. The introduction of Amy (Mayim Bialik) in the season 3 finale helped clarify him as heterosexual, though that label for Sheldon's sexuality will always have a giant asterisk next to it.
Sheldon's lack of interest or awareness of sex is what helped make one risqué joke in season 6's "The Fish Guts Displacement" work so well. His storyline involves Amy pretending to be sick in order to trick him into being more intimate with her. He eventually discovers the ruse, so Amy convinces him that some punishment is required. And so became the famous scene where Sheldon spanks Amy, oblivious to the fact that Amy doesn't consider it a punishment at all.
It's a scene that could've been creepy, but it works because of how sexless Sheldon (and to a slightly lesser extent, Amy) has been throughout the entire show. It was tough on the actors, however, in part because they couldn't stop laughing the whole time.
"I would say it's one of the hardest things I've ever had to do because I found it tremendously amusing," Parsons said in a 2013 panel. The scene was originally supposed to be filmed off-camera, with a shot of Amy's monkey reacting to the sight serving as the scene's punchline, so the last-minute change complicated things a bit.
"I was pissed, because I thought 'Well damn it, I would have practiced how to control myself if I'd known all week,' but all week I was slapping that ass off camera," Parsons said. He added about Bialik, sitting beside him, "You have no idea how game she is."