What I always found terrifying about zombies was less the horde, per se, but more the version of the myth that's more in the style of TVTropes' The Virus, where the victims become part of the horde. That whole sense of "your loss is their gain" and/or "they can make you one of them" is frightening on its most basic level - it's the corruption of you into something that is not you. That scares me in ways I can't even describe.
There's a similar sense of that around vampires, too, though it's tempered, I think, by the knowledge that in most (but not all, of course) variations on vampirism, you may change, but you still keep your own mind. But in truth, yes, vampire mythology is inexorably intertwined with sexuality. I didn't really come to understand this until I was in college, but yes, speaking as someone who had an ex-fiancee who was way too into this stuff, it's a big part of the vampire mythology. I don't know how much it plays into the current explosion, given that I've avoided Twilight (the source of the current outbreak) like, well, the zombie plague, ever since trying to read it made my wife weep at the heinous massacre of literature that it was.
