That's how I like erotic games too, a few possible love targets, and you choose which one the MC will be sexually involved with until that female character path ending. But in games like "Oh Daddy - Sara" or "Lewd Island" where the main couple are clearly defined from the beginning, it makes sense that the main couple should be the focus of the entire game, side characters would have little content, and the game would end in a specific situation involving only the main couple, as a result of your choices.
Maybe, if devs wanted to expand the game once it is finished, they could add different endings with other female characters, creating a path for each. But being realistic I don't think devs would want to touch a finished game they have under their belt, and basically creating a new game for each female character in their finished game.