<P> On the spur of the moment in "The Recombination Hypothesis", Leonard asks Penny out on a date after he imagines what getting back with her might be like . In "The Beta Test Initiation", their date goes well until Leonard asks about the status of their relationship . This causes conflict, due to Penny's subsequent admission that commitment frightens her, but they agree to take their relationship slowly . On a spur of the moment while having sex in "The Launch Acceleration", however, Leonard proposes to her, causing Penny great distress . After discussing the matter, Penny declines his proposal, while affirming her feelings for him, and her desire to remain with him . In the season 6 episode "The Date Night Variable", Leonard wants a reluctant Penny to define their relationship . In "The Decoupling Fluctuation", she wonders if she is really in love with him, but cannot bring herself to discuss it with Leonard . In "The Date Night Variable", Raj shows up unannounced at a private dinner of Penny and Leonard's, and inserts himself into their discussion of where their relationship is headed . When Leonard complains that Penny has yet to say "I love you" to Leonard, Raj attempts to get her to say it, causing Penny to eject him from the apartment . </P> <P> In "The 43 Peculiarity", Leonard becomes jealous of Penny's classmate, an attractive Englishman . At the end of the episode, Penny and Leonard discuss this, during which Penny casually mentions that she loves him, which Leonard notes is the first time she has said this to him . In "The Egg Salad Equivalency" Penny learns that Sheldon's attractive assistant Alex asked Leonard out to dinner and eventually admits to Leonard her own insecurity about their relationship . After a major argument with Sheldon, Leonard suggests moving in with Penny in "The Spoiler Alert Segmentation", but she indicates that she is not ready for this . In "The Tangible Affection Proof", Leonard again tries to propose to Penny on Valentine's Day, but she expresses that she is not ready for this, and is unnerved by that level of commitment . They resolve that when she is ready to get married, she can propose to him . In "The Bon Voyage Reaction", Penny deals with Leonard going off to Europe for a summer science assignment, resolving that they are more secure in their relationship . </P> <P> In "The Hesitation Ramification", Penny, inebriated and frustrated over the state of her acting career, proposes to Leonard, who is hesitant to answer, given the circumstances . Though this further frustrates Penny, she admits in the following episode, "The Occupation Recalibration", that Leonard did the right thing . </P> <P> By season 5, Penny has formed strong friendships with Doctors Amy Farrah Fowler and Bernadette Rostenkowski, and the three are often seen spending time together . However, Amy and Bernadette occasionally prefer to meet without Penny, and often tease her incessantly . </P>

Who does penny sleep with on the big bang theory