A lot of mainstream games in 2000s used to have sexual content - Witcher, Ghotic, Dragon Age, Fahrenheit, Dance With Rogues, Arcanum etc as well as less known, like all flavors of strip pokers, strip chess, strip match three... I guess first time i encountered sex in the game was some early-2000s adventure game, where you found yourself in hospital and have QTE sex with a nurse.
The first explicitly porn game was a visual novel about sisters, who had shared feelings. Like when one is horny - all three becomes horny. This is also the only game that I actually bought on a CD. Idk how that disc ended in that specific shop, but I was VERY curious about the topic.