Probably because people have a lot of understandable curiosity about it, but it's not something you sit and chat about casually at work or over a family dinner. Even among friends, it's often not discussed, or friends' experiences are all similar. It's a good place to comfortably talk about things that might be outside the comfort, manners or even morals of a person's real life environment.
But there don't seem to be as many people who want to sit around and discuss literary topics. There are more on the individual mangas' comments, but they're not often in-depth, objective discussions. I guess people come here not to think that deeply?
A site with potential like this, for example MyAnimeList is more then just talking about sex and/or related things, a good series isn't measured in just appearances alone, good storytelling, characterization and uniqueness are also key, artwork is good but it can only go so far as to actually represent a character's personality rather then just their bodies alone.