Not every pron game, but most people aren't exactly playing pron games with two hands for the plot... although putting even a little time into a game's plot is always a good idea, a game without any narrative is basically just solitaire. But yeah, I get it, different strokes for different folks (pardon the obvious masturbation pun!).
Beyond just graphical issues, having multiple breast sizes would also increase dialogue/written complexity. However, also easier to solve; can write code to apply modifier adjectives to written dialogue/descriptions, but also requires much more bug stomping so random code doesn't show up or wrong vocabulary chosen, etc.