CNN:

People describe the app using female pronouns. Her gender has even prompted some users to flood blogs and online forums with sexually suggestive questions for Siri such as “What are you wearing?” (Siri’s baffled response: “Why do people keep asking me this?”)The fuss over Siri’s sex also raises a larger question: From voice-mail systems to GPS devices to Siri and beyond, why are so many computerized voices female?

Siri isn’t universally female, either – Siri’s voice is male in its British and French implementations.

Apple, for its part, isn’t saying why Siri’s apparent gender assignment is as it is. But experts suggest that people are hard-wired to find female voices more pleasing than male voices, and many associate female voices with communication and relationships.