Jealous Kaho is so cute!
Though, I hate to be that guy because it's a hot and kinda wholesome chapter in the middle of a potentially awful arc, but this shouldn't be the first time she sees Shirasawa's cock. If he was always meant to pack this much heat, she should have ample opportunity to know by now.
She grinded against his cock to completion, when he was handcuffed to his bed, so she had ample time to see and feel his outline.
They also jerked off, fully nude, in front of each other, in that onsen. Pretty sure he was literally standing up in front of her, so she had his crotch at the level of her face.
But anyway, what matters is that this test was meant for Shirasawa to lose either way, as in, they both won. Unlike what she said, if he had managed to remain flacid, instead of being reassured, she'd have been hurt that her teasing him and toying with him left him cold. She just wants to reclaim her man because of her sister's attempt to seduce him away.
I'm happy it spured her to get bolder because it was starting to be a while they didn't toy the line that much. This manga is at it's best when both can't tell if they're just straight up fooling around or 'working'.
Also, the other love interests all have a head start on our heroine now and that's unacceptable! The people demand, at least, a blowjob next chapter 😤
Even better if she's smart enough to only edge and drive him, to set herself apart from Touka and make sure he's back to have her at the foremost of his mind