My first one was a mainstream one for business - avalon-data.com. It was back in '96 or '97 (what's shown in archive.org isn't my site).

My first adult domain was rawtop.com which I registered in 2001. My big mistake was not taking porn seriously back then. I had just come off a really profitable contract programming job working for Amgen, and 2002 was dead for me business-wise (thanks in part to 9/11). I had almost no work that year until Thanksgiving. Had I used the spare time I had on my hands to build porn sites I would have a pretty major site by now. Oh well... Hindsight is 20/20...