Until the Full Moon Vol 1

Marlo has a problem. On the night of the full moon, this half-werewolf, half-vampire undergoes a mysterious and terrifying transformation: He turns into a girl. Desperate for a cure, his parents call on Doctor Vincent, a long time family friend. But Marlo wants to keep his condition secret from Vincent's son, the vampire playboy David. Unfortunately, the secret gets out, and a new problem surfaces: David is interested in Marlo's female form! If a remedy can't be found, their parents believe the next best solution is marriage - a marriage between Marlo and David!

Source: Anime News Network Encyclopedia