Wow, Sol, talk about épater la bourgeoisie!
You go, guy. Don't hold back
I really do think you are making a good point in insisting on the equivalence of friendship and love. In light of the politeness and verbal reticence that characterizes Japanese culture, expressions of friendship can be forms of romantic discourse, so that couples can form and communicate all without ever having used what we might consider romantic language.
Practically, this means that entirely prosaic reasons suffice to construct a relationship. Traditionally, this is how marriages have always worked, where they are based on some practical necessity rather than romantic feeling. The idea of marriage as a romance is very much the late historical exception.
So when you ask "Is there any real reason for Chihaya and Arata to actually be together romantically just because they both love karuta?" I think you are missing your own point. The shared passion, commitment, and intensity, along with the shared social circle, activities, and organizational participation, that they share as exceptional karuta players is more than enough to justify a lifetime commitment to each other.
Of course, if this is true it does mean it is impossible to distinguish friendship from romance. Which proves my point: this is our exact experience while watching
Chihayafuru. Looking at Chihaya and Arata, we cannot tell the difference between friendship and love.
Kirarakim, this also means your argument that Chihaya's phone call to Arata can be interpreted as friendship rather than romance does no work. Just considered as friendship the call is intelligible as romance.
It turns out the problem we are having figuring out where Chihaya, Arata, and Taichi stand is not a matter of somehow plumbing the hidden depths of Chihaya's heart, but is instead the effect of a social ambiguity between the Japanese cultural concepts of love and friendship.
Brilliant post as usual Sol. +a billion.