How PR influence the result is a great mistery, people can only guess
The one thing is right, IMHO, that PR is just a mathematical prediction of probability that surfer will appear on the page, nothing more. Read the theory of Markov chains, you'll find lots of things that really reflect all the web. Google's algoritm of assigning relevancy is very difficult, and it take account of PR, but it is not even 50%. The top position by the word "porn" you'll have, if the googlebot will appear on your site by the link "porn" and find the word "porn" the most relevant on the page(how - it is another speech). I think you need aproximately 10^6 - 10^9 backlinks "porn" on your site to be surely the first in google by the word porn