The appeal of CGDCT is not singular. Innocence is certainly a part of why girls are chosen. But also, it needs to be cute and to most only girls are cute. Neither is the innocence universally applied and there are often not so innocent girls included in the cast. For example Konata in Lucky Star says some really not innocent things yet she is a popular moe character.

In the end, dudes rather looking at girls than at guys is still an important factor I think. Somehow being straight makes you like girls more even if you are not romantically or sexually attracted to those girls.