Yeah! I'm so happy that this thread is so wonderful! And I'm definitely glad that this didn't turn into a fight...
Anywho... I just finished a MASSIVE paper, so I'm in English-teacher mode. And I thought I'd add my two cents in, as to why I think KuroxFai will become an official couple.
First, as we've talked about recently on the KuroxFai Love thread, these two are shown together in Clamp's artwork a lot. And they are very close to each other physically in these pictures. Plus, as has been mentioned before in this thread, the manga has many pictures of Fai hanging on/off Kurogane and touching Kurogane in ways that Yuki and Touya (teh cutest couple ever besides KuroxFai) never were shown doing.
Plus, the way that Fai teases Kurogane is not the same way that friends and family tease each other. Friends and family generally tease each other by trading insults, like Touya calling Sakura 'monster.' That type of teasing is supposed to reinforce the relationship. It says, "we're close to each other, so it's alright if I say these things about you. We trust each other."
Fai's teasing of Kurogane, however, says, "HEY! I think you're pretty sexy and cute and all-out amazing. I think I'm pretty cute, too, so will you pay attention to me?" That kind of teasing is called -flirting-.
To get into some literary analysis (yes, literary analysis of manga), these two are very often isolated from others and forced to be together in several ways. In Shurano and Sharano, when the group was split, it was split Syaoran and Sakura, Fai and Kurogane. They were literally isolated from others they knew.
In a relationship sense, too, they are isolated. Fai and Kurogane are the two of the original characters in the series- every other major character they have run into has been a crossover or a person that would never be a romantic interest. The crossovers almost always have their own romantic interests and histories to deal with. Fai and Kurogane have no one else with whom they could have a romantic relationship.
Ashura is a possible former lover of Fai's, but I hardly think that they'll be getting back together after Ashura awakens, considering that Fai (a)froze him and (b)is scared shitless of him. I don't think Fai and Chi will have a romantic relationship, because Chi seems too childlike and naive. Plus, Chi and Hideki were together in Chobits, and in all the crossovers Clamp has done in Tsubasa, it has yet to break up a canon couple in one world or series to stick them with someone else in another world. If Chi ends up with anyone, it would be with a different version of Hideki.
As for Kurogane, I read the relationship between Tomoyo and Kurogane as a brother/sister relationship, not a romantic one. Besides, as I said before, I claim Tomoyo as a fellow lesbian and I refuse to let her go.
KuroxSouma won't be happening, because Souma is in love with Kendappa.
That leaves the two of them alone, with no possible romantic relationships, partnered together.
Plus, they are opposites, and Clamp loves to make those 'opposites-attract' relationships.
Also, I want to say a bit about the infamous aliases of Outo. Fai, rather than coming up with four completely different nicknames reflecting their four completely different personalities, purposely came up with 'Big Kitty,' 'Little Kitty,' 'Big Doggie,' and 'Little Doggie.' These four names are all opposites and paralells: Dog and Cat, Big and Little. Fai, by choosing these names, purposely compares him and Kurogane to Sakura and Syaoran. He is basically saying that, not only are Kurogane and Syaoran similiar and he and Sakura similar, but that, Fai is to Kurogane as Sakura is to Syaoran. If that's not romantic subtlety, I don't know what is.