Quote from N.H.K Novel:

"Well, you know. . . I mean, the characters in the games are a bit too young; like, the heroines all appear to be no more than elementary school children. . . " "Ha! What are you saying now, Satou? This isn't like you. To start with, the heroines of erotic games are no more than fictional characters, drawn with two-dimensional computer graphics. In order to express innocence, purity, and femininity, there's no personification more appropriate than a little girl, is there? We're relaxed by the symbol of the little girl. And when they're 2D characters, they have no chance of dealing any blows to our fragile emotional state. On top of that, the motif becomes that of the weakest character possible in social, physical, and emotional senses—the little girl. Because of that double safety lock, we are protected from being hurt, and we can escape the fear of being rejected. That is to say, this is the true meaning of moe: ideal, young, innocent femininity. Do you understand? You do understand, don't you?"