It's a "hatching chick" screentone — my best guess is that it's meant to convey the cutesy innocence of Kasumi's voice to contrast with what she's saying.
......?
What's wrong?
Does it... not feel good?N-No
That's not it...Mrrrgh...Frustrated that she was left out & impatient*Sob...*EheheWhy did you drag me into this...?I'm not very good at this, but I'll do my best ♡Ah...
Yuu-san does this... to you...?I'm just mimicking what Yuu-chan does to me all the time though...Eh~~~
This is your first time, Shioko~?It can't be helped~
This adorable Kasumin will teach you a lot of things ♡Y-Yes
Please treat me well, Kasumi-san...!Ayumu-san...K-... Kasumi-chan, this is embarrassiiing...There's nothing to be embarrassed about, Ayumu-senpai!!If you can't do something as simple as sex, you'll never be a first-rate schoo〇 idol!...Wait, you've already started!?How is it, Shioriko-chan?Yah... ♡Ah ♡
