It's an interesting thing to think about, but it's not really a point that you can use to take away a U.S. citizen's right to own a gun. As I understand it, the Constitution isn't saying "you must meet certain requirements (ie, join a militia) before owning a gun." It's saying "therefore no one's rights to own a gun shall be infringed." In other words, it's just giving a reason for outlawing the infringement - whether people actually join a militia or not afterwards is up to them.
But it's good to see a meme like this. It seems like there have been a lot of pro-gun ownership memes on imgflip, especially lately. I think it's good to have a meme that shows another side.