CTD (Constitutional TransSexual Disorder)... a rare condition that causes biological sex change.

Koyuki is a boy who has an appearance that often gets him mistaken for a girl. One day, his body transforms into that of a girl overnight, and now he faces the problem of the boys around him giving him perverted looks...

Whether it was due to Koyuki's status as a so-called "former boy" that they were being way too familiar with her, the fact is that the boys just keep begging her for perverted things.

"I beg you, Koyuki-sama! Let me see your boobs!!"
Koyuki gets swept along into crossing a line with such a blatant and straightforward request.

"You keep saying no, but..."
"Just enjoy it now that you've turned into a girl"
Koyuki is once again approached for sexual favors by a different boy who heard the rumors, and little by little Koyuki's resistance is overcome by the experience of a woman's pleasure...

"Was I this happy before over being called cute...?"
"Did that part of me become that of a girl, too?"
"Somehow, I feel a little... excited."

(Source: DLsite)