After their first sex, his girlfriend broke up with him as she was a lesbian that was attracted to his feminine face. And the only way for him to continue their relationship is to have sex while cross-dressed.
Included one-shots:
Volume 1: Koimonogatare: Tsumi to Batsu to Koi
Volume 2: Koimonogatare: Kuruu Hana
Volume 3: Koimonogatare: Naked