All American

"So, why is okay to sell mature games to kids but wrong to sell them porn? If the first amendment excuse works for games, why doesn't it work for pornography? How are they different?"

It's very simple. Sex and nudity are the devil, and will cause irrefutable harm to all involved. They are both completely unnatural, taboo, and should never be discussed or viewed by any decent people. Violence, however, is completely natural, moralistically acceptable, and will not likely harm anyone. That's the American way.