Pornography was banned from being sold to minors because it violates obscenity laws, or something crazy like that.
And in that single statement you point out the weird dichotomy that affects America where sex = bad but violence = good.
It's not a constitutional argument really is it? It's simply been decided that since sex is bad kids shouldn't see it but violence isn't bad so it's okay. If preventing kids seeing violent movies and games is a violation of their freedom of speech then preventing them from watching porn is too. I can't see how you can make any constitutional argument that distinguishes the two.
That's not to say that I think it should be alright to sell little Billy the "film" Bump Uglies 72, because I don't. By the same token, I don't think little Joe should be playing the game Manhunt: Vengeance. But it still violates The Constitution to ban both of these mediums.
Banning them from adults, yes. Banning them from kids. I'm not so sure about that. The simple fact is that porn is banned by doing an end run around the Constitution and claiming that it is obscene to sell a kid porn. Well if that's legal then why not simply say that selling a kid a violent game is Contributing to the delinquency of a minor or somesuch and be done with it? If you can say porn is obscene you can say graphic violence is obscene too.
Of course, the main responsibility for a child lies with said child's parent(s).
Back when I was a kid, I was physically punished if I did something wrong. I haven't shot up a school, or gotten into fights. I've never done drugs, smoked cigarettes, or anything of the like. Do you want to know why? It's because I was raised properly. I was raised to say "please" and "thankyou." I was raised to speak to my elders with respect. I was raised to obey the law, even when the law was wrong. And I think I turned out just fine.
Once again, The main responsibility for a child lies with said child's parent(s).
Who said it wasn't?
However just because the main responsibility lies with the parents doesn't mean that everyone else has the right to ignore their own responsibility. Just cause a parent makes the decisions on whether their kids will learn to shoot doesn't mean any gun shop can sell kids guns and leave it up to the parents to remove them from the child if they don't want their kids having them. Same goes for porn, cigarettes, alcohol, tattoos and a whole list of other things that kids often want but parents don't want them to have.
So why make an exception for violent video games?
One reason why I'm completely against government regulation of games, is that it will change what developers will create. E.g., If violent games (M+) are banned for sale to minors, then developers will almost always attempt to neuter their own work. The reason why they would do this is because they need to sell as many games as possible. Most modern games that are released by major publishers cost so much money to create, that over 200,000 copies have to be sold to break even.
Hang on a sec. You're saying that the reason you're against government regulation is because at the moment companies not only sell to children ignoring the M+ rating they themselves have set but that they only design games knowing that they plan to ignore the rating and that if they were actually forced to pay attention to the voluntary code they currently only pay lip service to they would lose sales?
Do you not see something fundamentally wrong with that argument?

The best example of self-regulation would be Soldier of Fortune. SoF had great violence blocks, as well as parental controls.
Except that parental controls are useless if the minor is the one who runs the system. If only the parent could buy the game then it's his choice to ignore his responsibility to turn them on. If the child can buy the game the parent never gets the choice.