It's news that won't be remotely shocking to anyone that's been paying close attention. But Seth McFarlane has gone on record about Stewie's sexuality: he's gay.
In an interview with Playboy, the Family Guy creator said:
"We had an episode that went all the way to the script phase in which Stewie does come out, [but] we decided it’s better to keep it vague, which makes more sense because he’s a 1-year-old. Ultimately, Stewie will be gay or a very unhappy repressed heterosexual. It also explains why he’s so hellbent on killing [his mother, Lois] and taking over the world: He has a lot of aggression, which comes from confusion and uncertainty about his orientation."
Again, any loyal Family Guy viewer, such as the one that compiled the video below, figured this out ages ago...
[video url="http://www.tvfanatic.com/videos/gay-stewie/" title="Gay Stewie"] [/video]