It's good that you contacted them, but have you ever heard the axiom "You'll catch more flies with honey than vinegar"?
I think your letter would have more impact if you were less abrasive and presented your rebuttal in a calm and civil manner.
For example, don't call them freaking morons and raging mothers. And mentioning 'female emancipation' (in the 80's?) really makes you look ignorant. In fact, the opening sentence alone would only serve to add more fuel to their arguments; you're obviously a gamer and look at how you react.
The second paragraph is much better, but again calling them evil won't get you anywhere. The third paragraph is much better because you're backing up your statements. However, calling American society screwed up, again, doesn't help. Also, the word you wanted was 'nonexistent' - make sure you clear this stuff up before sending letters like this out, people have a field day with that.
The third paragraph is kind of weak. Ignoring the misused words, the statement is pretty loose in and of itself. Providing some examples as to why their arguments are wrong would help here. Tell them where they're incorrect, and correct them, rather than just slamming them with another insult.
And again, at the end, a bunch of insults. In fact, the letter makes you sound something like an irratrional, very angry young man - probably just what these people made gamers out to sound like. I wouldn't be surprised if they used your letter to bolster their arguments.
The best thing you could probably to to actually HELP the situation and educate these people would be to construct a knowledgeable, well written, polite letter, and send it to them with an apology for this one.
edit: hereis a quote from their main page: "We have received an overwhelming amount of letters on the topic of videogame addiction and violence. While the majority of the responses were sent in by feral young children and pre-teenage gamers..." now just think about your response to them.