Ryuki had been texting a lot with Asuka at night, and most of their conversations revolved around erotic topics. Asuka revealed that she had previously watched adult videos, but only of futanari or girl-on-girl content. She had never seen a male and female adult video before.
That changed today during class when she watched one for the first time, and Ryuki happened to catch her in the act. Intrigued, Asuka began bombarding him with questions about erotic matters. As a result, Ryuki found himself staying up all night answering her endless stream of curious inquiries. They also ended up reading erotic manga together, though they were each in their own homes—Ryuki in his, and Asuka in hers.
The situation worked out conveniently for Ryuki because his mother, Fumiko, had gone to bed early. Taking advantage of the quiet, Ryuki had called Rena, asking her to invite Saki over to her place, ensuring he wouldn't be disturbed for the rest of the night.