The Vacation Solution - S5-E16
Howard: I'm not signing a pre-nup.
Penny: Alright, Howard Wolowitz, listen up. You sign anything she puts in front of you because you are the luckiest man alive. If you let her go, there is no way you will be able to find anyone else. Speaking on behalf of all women, it is not going to happen, we had a meeting!
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.