It's not really the pose that did it. Senku looked really good as a bunnygirl. Maybe that was only Gen's opinion, but he thought it was the one that counted.

But that speech. Gen started to giggle, quite a bit, losing his composure entirely.

"I'm sorry, Senku-chan," Gen said in between his laughter, "sorry, it's - okay, let me try again. Well. Do I really have to apologize yet? I don't feel particularly sorry, after all~"