Well, the one who had the decision made for him didn't fit, that was the point. Even in a world where you can choose your own sex, mistakes can be made, especially if you can't decide for yourself.
Floe choosing to become Floef is out of left field, but as he said, it was a spur of the moment thing. It really didn't matter which sex she lived the rest of her life as so she did a mental coin toss and ran with it.
And if you're missing the character development in Marimite, go back and pay more attention. It's subtle and slow, like everything in Marimite, but compare the relationship Yumi & Sachiko have at the end of the second season with where they are at the start of the first.