One of the most persistent themes of modern fantasy literature set in the modern world is the theme of sex with the supernatural. The asexual nature of vampires has been one of the refreshing differences in Ilona Andrews' Kate Daniels books (Magic Bites et cetera) - of course, lycanthropes and shapeshifters are oh-so-sexy.
There are, of course, patterns within the details. I'm having trouble thinking of many leading pairs in which a female vampire and a male human connect together; much more often, it's the other way. The Librarian: Curse of the Judas Chalice comes to mind, but primary romantic arcs usually feature a female human and a male vampire, as in the much more widely watched contemporary film Twilight. And when I move to novels... the only cases I can think of where male human and female vampire pairings are exhibited are in secondary characters.
If anything, I think I've seen more male human/male vampire pairings in that type of literature. But when we shift over to werewolves... the same pattern isn't so clear. Werewolves in popular "modern" literature persistently feature both female and male werewolves, and it isn't unusual to pair a male human lead with a female werewolf (An American Werewolf in Paris) or suspected werewolf, as in the brief (and quickly canceled) show Wolf Lake.
Maybe it's because lycanthropy is often described as a family curse, passed on through the generations, and wolves are so well known to be social animals; perhaps it is a consequence of the originating literature. In the case of modern vampire novels, everything returns to Bram Stoker and Dracula, often taken as a commentary on female sexuality in the Victorian era; the brief flirtations of Dracula's brides with Harker is a single scene, while Dracula's designs on Lucy and Mina occupy the central plot of the movie.