Well, if we can trust ff-xiii.net, this article:

Nomura Kotaku Interview: 'Noctis is Gay' - Final Fantasy XIII: News, Trailers & Screens ~ Final Fantasy-XIII.net

states Noctis is gay.

Interest in the game: -200% (And before you scream homophobe, people can be gay as much as they want, but that doesn't mean I have to like it in the games I play.)