At some point - either pre triplets or when the triplets were younger and the nishigoris had a rare night off from looking after them - takeshi, yuko, and yuuri had a threesome.
They were all friends after all, and yuko is quite honest about her finding yuri attractive, and takeshi is comfortable enough to admit the same.
Yuuri - whether seduced on the night, or aware of the plan - is reduced to a whimpering puddle of pleasure, and everyone has a good time.
(Only squick is any of them being underage. They were all young adults when it happened, ideally.)
Takeshi/yuko/yuuri, everyone agrees yuuri is hot, past threesome
They were all friends after all, and yuko is quite honest about her finding yuri attractive, and takeshi is comfortable enough to admit the same.
Yuuri - whether seduced on the night, or aware of the plan - is reduced to a whimpering puddle of pleasure, and everyone has a good time.
(Only squick is any of them being underage. They were all young adults when it happened, ideally.)