One of the first titles known for strong sexual content was this text-only adventure in 1981 by On-Line Systems where the player is out win the affections of three women:

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Softporn Adventure was a notable early example of a game with strong sexual content, released in 1981. It was a text-only adventure game where the player's goal was to seduce three women, distinguishing it from graphical games that came later.

One of the First Titles Known for Strong Sexual Content