
Calling me stupid...baka! (/TДT)/ I wasn't really sure that they were together or not because Mii-kun confessed before going to Tokyo and I didn't really get if the reply was a Yes or No(so, not sure), and then when they were already together in Tokyo it was specified that they were ALREADY(??) a couple having a long distance relationship. In opposition with your comment, which I'm glad you pointed it out, I'm actually confused to "when" did they actually become a couple.
Sorry if I can't covey this properly since english is not my first language.

Mii-kun confessed and they were together for a bit and then broke up. Mii-kun waited for Kei-chan for four year in Tokyo. However Kei-chan told him that he going to take a job in their hometown. Therefore Mii-kun thought it's already hopeless. He feel overwhelmed by loneliness and went sleeping with Kitahara and Haru. Whatever they didn't date back then when he did it so it's not cheating ╮( ̄▽ ̄)╭

actually kei chan was the one being mean, he said a lot of things were impossible would take no advances and basically made mii chan feel like it was hopeless with this straight man. Kei being without Mii was what they needed and eventually their relationship just progressed. Just because you like someone doesn't mean you cannot use your body as you will with another... its kind of a realistic feeling on that point.
I am bothered by the fact that Misaburou(Mii-kun) is most certainly not a virgin 'after' they(with Kei-chan) became a couple(?) I just don't like it that he actually CHEATED(most likely) on Kei-chan when he was in Tokyo. Mii-kun's personality, from childhood to adult, left too much of a wide gap for me to comprehend. He was such a sweet kid when he was younger!!!
That's it, that's it! As said by the author - "Even though he has Kei-chan, he is easily swayed by other people." That's what I don't like about Mii-kun. At least Mii-kun admitted that he slept with...'a few' guys, and Kei is a possessive, naturally sly boyfriend so he is perfect to keep a short leash at Mii-kun; it's all because of Kei-chan that Mii-kun changed~
With Kei-chan and Mi-kun so very loving and adorable in their own way after all that had happened, I guess I could forgive Mii-kun's past that gave me angst! It's hard not to love them, It's so sweet how they are becomeing closer and closer and more affectionate and Kei-chan being more assertive and Mii-kun being all bluuushies~
Forget what I said! The chapters just keep getting better and better! Also, I'd like to see them reverse positions at least once!