You know that mankind has reached a new level of evolution, when Science finally coincides with Porn... Must say that I was rather stunned when I first came across the video
I must say, that I was quite skeptical about the truth in it, and that it might have been a spoof... but then... i chanced upon the interview on Improbable.com
And the finally, on googling... yes... there indeed has been a paper published on it... in the British Medical Journal in 1999... titled Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal.
Truely, this is as naked as you can get.