However, I have read fic where Spike was having sex with Buffy or Xander or whomever (well, other than Angel or Dru) where the human counterpart was watching a mirror as they were being penetrated (heh - sorry!) by, well, nothing; an image I found terribly amusing while I'm pretty sure the writer was going for guh/humina/hot.

I've done the mirror thing a time or two, but it's not because I find it particularly hot (I have no feelings towards mirror sex one way or another). It's because in the show, when they did it with Angel and Buffy, it was always to underline that Angel didn't belong in her world--didn't even exist in her world. So when I do it, it's usually in a context (not always sexual) where I'm trying to reclaim and reinterpret the no-reflection meme to be inclusive rather than exclusive--show that while the vampire may not be visible, they are an agent in the visible world nonetheless.

That sounded a lot less pretentious in my head.