I stumbled into it completely by accident a couple of years ago. I have no idea what the hell I was searching for at the time, but somehow I landed on GFY.com, started reading threads and thought to myself, "Sell porn? Damn, I can do this."
So I started promoting gay porn and was doing pretty well with Adwords at one point, then Google slapped the shit out of my campaigns. And I guess I just decided to try some mainstream stuff (wow gold LOL) and within a couple of weeks it became clear that making money pimping mainstream crap was about 10 times easier than making money pimping porn.
So I abandoned the porn racket. All I do now is renew those domains. They still make me a few hundred bucks a month from search traffic and revshare every month, but I don't work on them anymore. Though lately I've been thinking I should take a couple of days every month to put up some galleries or blogs because revshare is sweet money and it doesn't seem all that common in mainstream affiliate marketing.
When I started all this, I was still working a regular job. I ended up quitting for other reasons, but at some point decided that I'd just rather do this full time so that's what I do.