So that tangent finally got its own story, eh? Glorious!
To those of you who don't know the series, in Nisemonogatari episode 8, Karen (yellow jacket) asks her brother (the guy here) if she can meet up with Kanbaru (the other girl) whom her brother knows and who's a fellow sports fanatic (and die-hard lesbian which Karen doesn't know).
Karen's brother is not overly fond of the idea but then makes a bet with her, that if she can withstand him brushing her teeth for 5 minutes, he will agree to letting them meet up. Karen thinks that it's an easy win but is in for a big surprise, when someone intrudes her body with an object which caresses an area in a way that most probably find rather dull but which is far from the truth when done by others.
The remainder of the episode takes on a rather sexual undertone and ends with Karen winning the bet although she has had to struggle considerably with moaning and giving in to the brushing along the way.
They never show them meeting up so the author has taken on that liberty, but I think we are many who will not complain about this.
The toothbrush incident itself has got its fair share of doujins already, though.