Meet David. His father's a count and a vampire. His mother's vampire. David is, therefore, a vampire. (Forget traditional vampire lore-it only holds true in these stories when we want it to.) Meet Marlo, David's cousin and childhood friend. His father's a vampire. His mother, however, is a wereworlf. This makes Marlo a... throwback. When exposed to the full moon, he turns into ... a woman. 

His mother and father have brought him back to the family estates seeking help, after a 10 year or so of absence. what they come up with, since each family needs an heir to carry on the line, and since neither of the mothers can bear daughters for some reason, is that David and Marlo should marry. David is more than happy about this, since he's bee lusting after Marlo ever since he laid eyes on him again. Marlo's not quite so sure...he still remembers all the nights they shared a bed when they were kids, and David always sucked his blood when he was asleep. He finally agress, but absolutely refuses to do any kissy-face stuff unless he's a girl-MUCH to David's dismay. David doesn't give a hange whether the body's male or female, as long as it's Marlo. And there's only a full moon for, what-at most 5 days in the month? A charming story, and this time, 

The author has also done a short dojinshi-in which David finally convices Marlo that gender isn't important.  You can read an excerpt in the comiket section.