A second sex, Dom/Sub, emerged in the evolution of mankind.

Doms dominates Subs and Subs are subordinate to Doms.

Watase, who is a "sub but top," distanced himself from sex from play due to the trauma of being being almost forcibly dominated by his Dom lover in the past.
One day, he quickly went to a shop that specialized in sex toys in order to satisfy his lust.
When he confesses to Touma that he is a submissive top… he gets an unexpected reply, "I'm a dominant bottom”…?!