It's a Threesome, Got a Problem with That?

I am Zenin Zi. When I was eighteen, I let my childhood friend enjoy a three-person relationship under a one-husband-one-wife system. No, calling them people isn't quite right, because... “Rika, your hair is perfect for playing.” “Zi, don't teach Rika bad habits...” “Yuta... do you... not like it?” “If you don't like it, Yuta, you can try my Ten Shadows Technique~” “You two have had enough!!!” Theme: I want to write something only perverts would understand.