I voted for "created with real gender and sexual preferences" if it's what I think it is. I think most Clamp characters seem to be created with the 'someone just for me' type of idea regardless of orientation, and Clamp doesn't seem to care about whether they're gay, straight, bi, or...nothing. It's sad that people still have an issue with gay people/characters, but whatever. That'll pass eventually, I hope. Unrelated, I don't think Fai could possibly be any gayer if he tried. XD
In most cases the orientation isn't even mentioned, as if it should just be accepted. In a few cases they make a bigger deal out of it (Tokyo Babylon) as part of the story. I don't think all the characters are necessarily gay, bi, straight, genderless, asexual...etc. I think there's a mix of just about everything in every story that Clamp has written.
People will always see what they want to see though, so if they want to pretend that Tomoyo only loves Sakura in a sisterly way, nothing will probably convince them otherwise. Funny how we don't have to argue that Sakura's straight though.