Minami-kun no Junan ~Kyousei Seitenkan Gorankou~

The protagonist, Minami, had been living a fulfilling life with his girlfriend, Mari, enjoying a healthy sex life.

One day, Mari had to leave for a training camp and asked Minami to look after her younger brother, Kaoru.
Minami had always felt uneasy around Kaoru, but he couldn’t refuse Mari’s smiling request and reluctantly agreed.

The next morning, however, Minami woke up to find his body had transformed into that of a woman!
Shocked, Minami confronted Kaoru, who confessed to slipping him a “gender-swapping drug.” Before Minami could process what was happening, Kaoru pushed him down—or rather, her—and took her virginity.

As Kaoru continued to “develop” Minami’s newfound female body, Minami found herself growing addicted to the pleasure. Soon, she was indulging in sexual encounters with younger boys Kaoru brought over, losing herself in the sensations…

[Translated from dlsite]