There are many reasons for a skilled fighter to not dodge something coming straight at him.

Koutarou picks "it'll be funnier that way" and headbutts the volleyball back, sending it up, up and away back towards the ladies.

His face is flushed, but not from the sudden headrush. The fact is, you don't get to see girls in bikinis in Soul Society much...

...especially not THOSE bikinis, hubba hubba.

(He could've pulled a Vash the Stampede here, but not a lot of people are old enough in this forum to spot the reference.)