Incest love. Sin and innocence at the same time.
Ayako was locked in basement when she was four. When she turned 15 she developed an undaunted sexual interest towards her brother Shiro. They used to cohabit every time Shiro came downstairs to deliver her rations.
The panel above depicts them in the moment after the orgasm, the so-called afterplay.
From Ayako by Osamu Tezuka