[This doujin opens with [Natsume] and Emu mid-conversation about Tsukasa. Unfortunately, they agree, he's a little oblivious, and might need even more encouragement to have this....... threesome? Oh. Oh gosh!

The artist of this particular doujin is clearly of the opinion that Tsukasa has never been with a girl before and will require both instruction and assistance from his partner(s). When Natsume broaches the topic, Tsukasa is clearly indignant, but it's very easy for Natsume to redirect that indignation into competitiveness. Come on, Tsukasa, let's see who can bring Emu to orgasm first...!]