That it wasn't anything to be "ashamed" of, culturally, doesn't change the fact that anal sex can be brutal on young tender skin. Sex with an adult forced upon the young via the cultural climate of the period in question cannot be justified in my book, whether it's homosexual sex or heterosexual sex.
Not everything they did in ancient greece was a good idea -- they thought having slaves was normal and women were not only not allowed to compete at the olympics but were liable to be stoned to death if they dared to sneak in as spectators. For me, their sexual preying of young boys fall under one of these "bad idea" categories.