Sheldon's relationship with Leonard and Penny in "The Big Bang Theory" is often a complicated one. While the show never actually names a specific medical condition for Sheldon, he has a tough time fitting in and understanding social situations. This puts various strains on Leonard and Penny's friendship with him (as well as on their own marriage), especially when Sheldon puts Penny down — which happens quite often.
In seemingly every episode, Sheldon refers to Penny in some sort of negative or derogatory way. He often implies that she is stupid, overly promiscuous, lazy, bad with money, and a poor actor. For example, in the Season 4 episode "The Robotic Manipulation," Sheldon misogynistically suggests that Penny has had 31 sexual partners, which clearly embarrasses her. He even goes as far as changing the Wi-Fi password to insult her, making it things like "Penny is a freeloader" (Season 4's "The Thespian Catalyst") and "Penny already eats our food, she can pay for Wi-Fi" (Season 5's "The Speckerman Recurrence").
While some of these are somewhat harmless jokes, that's not always the case, but even when Sheldon takes it too far, Leonard doesn't really step in and say anything. He occasionally mutters a word in protest, but never really tries to make much of a stand. It shows a stunning lack of respect for his girlfriend and later wife, and is another strange aspect of their relationship.
