I was just listening to this on the radio yesterday and i thought if 'once more with feeling' had happened even a few episodes later on (with buffy and spike at it like wild thangs!) this could have been sung by both.
Also the song is by Mundy this Irish guy who rocks! so if your able to download him for Kazaa or Winmx go for it!

Healthy - Mundy

Spike
Kiss me quick is what she said
Before they come back in
I tell you this and it is true
I've never tasted skin
So soft and smooth
And brown and soft
A tremor of the heart
Buffy
If anyone should find this out I could lose my part

Buffy
And this can't be healthy
And this isn't on
Oh no, no ,no
Spike
Ooooh but it is amazing
Or could it be . . .
Buffy
Because it is wrong


Spike
I hold my waist
And taste my taste
Bite the whole way through
Buffy
And I could sign my life away
For this to continue
And what to do with double edge
'Cos everyway I lose
Find a chair, a hook, a rope
And tie me up a noose.


Buffy
And this can't be healthy
And this isn't on
Oh no, no ,no
Spike
Ooooh but it is amazing
Or could it be . . .
Buffy
Because it is wrong
Its wrong

Buffy and spike kiss
Ooooh mmmmmmm