At first I thought Utsuho was doing this to herself and she was just bewildered by these pink things on her breasts and why it felt so good to pinch them, and then I saw Koishi's clothes and then I went from "laughing that a mentally challenged cutie is now discovering masturbation" to "sad because said mentally challenged cutie is actually being molested."
Could you imagine the mortified look on Orin's face as Utsuho excitedly ran to her and told her how awesome she felt when she touched herself?