Complete all routes with at least one ending, then start a new game and select the "Fufill The Promise" option to play the Bad Boy's Love route.

The following birds prefer the listed beans:

Anghel: Bitter Black Beans .
Kazuaki: Country Millet.
Nageki: Country Millet.
Okosan: Swollen Hemp.
Ryouta: Calm Corn.
Sakuya: Racing Blend.
Shuu: None, although Racing Blend will trigger various reactions.
Yuuya: Bitter Black Beans.