Leslie: Listen Leonard, neither of us are neuro-scientists, but we both understand the bio-chemistry of sex, ... I mean: Dopamine in our brain is released across synapses causing pleasure. You stick electrodes in a rat's brain, give him an orgasm button and he'll push that thing 'till he starves to death.
Leonard: Who wouldn't?

Leslie: There's no reason to feel uncomfortable just because we've seen each other's faces and naked bodies contorted in the sweet agony of coitus. Rest assured that any aspects of our sexual relationship regarding your preferences, your idiosyncrasies, your performance, are still protected by the inherent confidentiality of the bedroom.

Leslie: Hello men. Sheldon.

Leslie Winkle: Uh, in the past, I would've said something obnoxious, like, "Happy birthday, dumbass". But I'm not gonna do it. You and I have both grown a lot, and it's just so nice to see you all again. So happy birthday, Sheldon.
Sheldon: Oh. Oh, now, you know I hate change. Say it.
Leslie Winkle: Happy birthday, dumbass!

Sheldon: Who do I speak to about permanently reserving this table?
Penny: I don't know, a psychiatrist?

Leonard: If Penny didn't know that Leslie had already turned me down then that would unambiguously mean that she, Penny, thought I should ask her, Leslie, out, indicating that she, Penny, had no interest in me asking her, Penny, out but because she did know that I had asked Leslie out and that she, Leslie, had turned me down then she, Penny, could be offering me consolation. "That's too bad, you would have made a cute couple" but while thinking "good, Leonard remains available."
Sheldon: You're a lucky man Leonard.
Leonard: How so?
Sheldon: You're talking to one of three men in the western hemisphere capable of following that train of thought.
Leonard: Well, what do you think?
Sheldon: I said I could follow it, I didn't say I cared.

Sheldon: I don't know what the protocol is here. Do I stay, do I leave? Do I wait to greet them with a refreshing beverage?
Penny: Gee, Sheldon, you're asking the wrong girl. I'm usually on the other side of the tie.