Leonard: So, Penny doesn't want me around her friends. I embarrass her. What else could it be?
Sheldon: Well, her actions could be out of concern for your feelings. Perhaps she's excluded you from these gatherings because she's scouting for a new mate and doesn't want to do it in front of you.
Leonard: Oh, how kind of her.
Sheldon: Agreed. Most primates don't show that sort of discretion. A female bonobo will copulate with a new male in front of the old one without so much as a how do you do.