I think you have every right to put your foot down. My husband grew up with an older brother adn they were "all boy" and allowed to play the games and play with guns etc. I have fought tooth and nail to have such things not allowed in my house. My defense and argument: When he was growing up, it was a much different world, the most he practiced in school was a tornado drill, no one had ever yeard of the "shooter in the school drill"
My oldest son is only 5 but we recently discovered Cartoon Network and with in a few days of watching it I noticed a huge change in his attitude and the way he played with his toys and siblings. We have since blocked the channel and after some time he his back to "normal" for the most part.
Your husband playing the games is one thing: he is a grown man and understands what the game is: a game. Your son's are still growing and developing and although the know it is a game, it still changes the way they view things and the way they think. Yeah, the could see some violence on the news, but on the news you don't see people hunting other people down and shooting them, personally I don't even watch the news when the kids are around because of what they can see. But that said, I think kids are less likely to sit glued to the tv when the news is on, unlike a video game.
My two cents, for what it is worth: stick to your guns,pun intended :)
M.