The Desperation Emanation - S4-E5
Leonard: What about you, Stuart? You have a girlfriend yet?
Stuart: Oh, yeah. Yeah. I met her at Comic-Con, the one place in the world where saying I own a comic book store is an actual pickup line.
The Agreement Dissection - S4-E21
Sheldon: You may have gone to Cambridge, but I am an honorary graduate of Starfleet Academy.
The Zazzy Substitution - S4-E3
Sheldon: May I point out that for eight long months, I suffered in silence as your female companion filled our apartment with her off key country music caterwauling, the unappetizing spectacle of grinding a pommel stone against her calloused feet in our living room, and night after night of uninformative TV documentaries about the Jersey Shore.
Leonard: Suffered in silence?
Sheldon: Yes, and I would expect you to do the same.
Leonard: Really? SILENCE!?
The 21-Second Excitation - S4-E8
Bernadette: I really love Howard's chest hair.
Penny: Howard has a hairy chest?
Bernadette: No, just the one, but it's really long.
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.