Oh ye of little faith :) Actually, I have, in the past, been a registered Libertarian and, while no longer a Libertarian, have great respect for the many thoughtful Libertarians here on FR.
I think your and the other responses by Libertarians on this thread are encouraging and good. What I hear is that, no matter how consensual this looks, there are some things that are just too evil to allow and Libertarians are going to figure out a way to invalidate his consent, no matter what.
And that is good. There are some private, consensual things that are just too evil to allow.
What I think is worth asking about is what that says about Libertarianism on other private, moral issues.
For example, is this case really that different from Euthanasia. Stylistically, the victim's choice of how to die in the article was horrible. But isn't that form over substance? He chose to die and Dr. Kevorkian's victims chose to die. So Kevorkian's victims weren't perverts in addition to wanting to die. Is that a reason to say that Libertarians should treat the two cases differently?
Regarding drug use. I have seen it up close. A junkie has no more ability to act voluntarily than you think the victim in the article had. Both of them will eventually die of the very problem that produces the inabilty to act voluntarily. Why should Libertarians treat the two differently?
Similarly, with prostitutes. Most of them are very messed up young girls with a history of physical and sexual abuse in the family whose ability to voluntarily adopt their 'lifestyle' is dubious, just as you argue that the victims consent in the article is invalid.
What this really says is that conservatives and Libertarians really share more than the Libertarians usually admit in public. Both believe there should be some limit to the private, voluntary behavior of people. They just draw the line in different places.
Libertarians like to think the difference between them and conservatives on moral regulation is one of principle--the principle being 'no regulation of private, voluntary behavior.' But that is not really the difference. It is actually one of line-drawing--where is the line across which behavior is so unacceptable that it should be regulated. I would suggest that the line is not nearly so clear as Libertarians often think.