Gendou met Yui, they fell inlove, which left Kozo out of the picture.
And we all know what happens when one loses someone he/she loves to another person (Heck! Just check out Ranma 1/2 and you'll definately see examples of this from A to Z).
PART 2:
Bizarre Love Triangle
Naughty, naughty Fuyutsuki!!!
Chapter 3: Yui's gone...and is that Fuyutsuki's hand on my butt?!!!