I would say that Buffy isn't so much evidence that two opposed motifs can exist at the same time in a show, so much as that they can exist in close quarters, alternating in a schizophrenic-but-enjoyable fashion. I don't recall many moments where I was thinking of any of the characters as both teenagers and gothic-fantasy-world occupants (even in, say, The Body, you're just watching a well-plotted drama about a teenager; the vampires, though serving as setting elements, could be traded for mobsters or hospital patients or needy pets in that episode without affecting the theme.)