Sexymon Adventures

From the letters page: Sexymon Adventures: an adult game about romance, parody and MILF, developed by Winterfire. It runs on Ren'Py and puts you in the role of a male protagonist. Status: finished, version R6. Available for Windows, Mac and Linux, and playable in the browser on Xgame9; 55 MB.

From the box: “The protagonist (nameable) just turned 18 years old and goes to an adventure around Kanto to encounter all the monsters and research a special monster mutation for the town's professor.”