Today, a hint of an answer from the series' executive producer Chuck Lorre, at the Television Critics Association conference in Pasadena. Lorre's thoughts:
Part of his character, which I think is wonderful and unique, is that he has chosen not to play in the relationship game. Either way, heterosexual, homosexual, bisexual, any sexuality, he has said, "This is not for me. I'm a scientist. All I'm interested in is science and what George Lucas does." I think it's terrific that we've stumbled into creating a character who has chosen a lifestyle for himself that's unique. And I don't see any reason to modify it.
Ah, so that's it. Definitely sock puppets. George Lucas sock puppets.