I get a lot of emails asking this question, so here goes:
In the winter of 2000, I purchased IndiePorn as a joke. The original intent was to put up a satire of subculture pornography; it was an immature idea. Fortunately, time to work on that never materialized - but I decided to grab several variations of the "Indie" identity, including alternate spellings and .net:
Within a few months, offers to buy the domains started coming in. No one offered me a million dollars, ten thousand dollars, or even one thousand dollars. I've turned them all down, even to this day. Why? Because the highest offer I got for all domains was Five Hundred Dollars. Surely, one would think, the phrase "indie porn" and all its spellings would be worth more!
At some point you realize that investing in pornographic internet domains isn't like doing the same with business domains — someone isn't going to make a serious offer on the value of your portfolio.
So I kept the domains. About a decade later, I got a few offers under $10,000 - but turned them all down as they seemed too low to part with. Between the lack of ever getting a super serious offer, and the lone fact that I can say "Yeah, I own those names. They're just sitting there … like always", it seemed like the right thing to do.