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.

(Source: MU)

Included one-shots:

Volume 1: Koimonogatare: Tsumi to Batsu to Koi

Volume 2: Koimonogatare: Kuruu Hana

Volume 3: Koimonogatare: Naked