Ads, sex sells
! It is as simple as that... The only difference between anime
and other forms of media in this respect is the fact that it is animation!
Artists can draw whatever they like, they are not limited by what an actual actor looks like. So for instance in a live action movie, you might cast a really good actor, who happens to be slightly overweight, or has small breasts or whatever, but because he/she is such a good actor you cast them anyway. With anime you don't have that problem. You can cast the fattest, ugliest, most visually unappealing voice actor you want. They may not look good, but they sure can act. Their looks are unimportant, the animator just creates the character to whatever requirements are given.
Take Neon Genesis Evangelion, sure it offers fan service, it features two very well developed 14-year-old girls. Now I ask the question, if sex sells, and by its nature animation can be whatever you want it to be, why would you have two of the main characters being unattractive? Sure Asuka's breasts are quite large for someone of her age, but I don't think that is a product of it being Japanese animation more so the society we live in.
Look at any music video and you will be hard pressed to find a female without large breasts. Look at TV shows, like Dawson's Creek, the actors are in their early twenties, but they are playing 15 to 16 year olds. You should note that all of them are extremely well developed "15" year olds.
My point is don't knock anime. I don't think it has any more sexual overtures than anything else we are exposed to today, it is just that by its nature it is able to make all it's characters cute, or beautiful. I doubt you will find an ugly main character in anime, unless it's a plot device, but I don't see anything innately wrong with that. Is air brushing and photo manipulation on magazine covers any different?