The first serious adult game I played was Man of the House, barring some mobile games that I wouldn't count.
It's a really good game, I played to completion.
It depends on your definition. Like I was fucking around on Second life getting up to all sorts of mischief, also would play various horrid flash animation games.
But renpy style? DFD. Then Double homework. Then seeking leaked patreon chapters lead me to F95 etc
Summertime Saga was the first game I played because I remember I would be looking for porn games online before I found these types of forums and I was lucky enough to find this game first since it has its own website.