Gen squeaked and immediately squirmed, trying to twist away and unable to. With enough warning maybe he could've steeled himself - instead, Gen was caught completely by surprise by Senku.

"Ah! Sen - " he's also giggling - "Sen-ku-chan, not - not fair!"

Of course as noted before, he didn't have much range here - there's nowhere for him to escape to. Nowhere for him to leave from those merciless fingers. Gen was vulnerable, twisting helplessly beneath Senku.

Rubbing his ears had aroused him - he'd begun to get hard - this kept him there, albeit for different reasons. The same reasons? Gen couldn't spare a braincell to unpack this.

"Senku-" Gen's tone already was begging even if he wasn't actually asking.