At some point you start to wonder how they come up with this, but at the same time, I think it's the same as any creative process: just a bunch of dudes (and hopefully gals as well) gather and brainstorm:
"Say, how about we make it about a girl who has to be in front of a crowd?"
"Okay..."
"And she has to pleasure herself..."
"So what? Anyone can do that... We're SOD goddamn it! We have to make people go, "Those effing guys did it again!""
"Okay, okay. Hear me out. So, she's an employee in a suit... Wait, she will have to get undressed..."
"What if she's only naked from the waist down?"
"Nice!.. But why does she have to do that?.."
"Her bosses told her to?"
"Okay, but it's still too general..."
"What if she has to do it with stationery... Right, that's been used to death as well..."
"..."
"Death! What if she has to do it with a gun or something? A challenge where it's off the safety and she can shoot herself if she gets too carried away..."
"Too much! And also: a gun? In a company? It's Japan! No guns!.."
"Okay, what if she rides a bike with two dildos?"
"..."
"..."
"Perfect!"
But yeah, interesting to see how they actually explain the need for the bike with two dildos in the plot...