Where does he say it is? He said pay your taxes. He didn't say pay your taxes only if you agree with government.
Extortion is when you force somebody to give you something of his, even if he doesn't want to, under threat of violence.
Taxation is when the State forces somebody to give it something of his, even if he doesn't want to, under threat of violence.
Same-same--except that the State gets away with it, because it's the State.
Did you read the link? Jesus isn't saying that the government had a right to demand the tax under consideration. As a matter of fact, he's saying precisely the opposite. Don't miss verse 26. But he thought that paying it wouldn't do any significant damage, and refusing to pay it would lead to unnecessary and unproductive trouble.
"But whether the Constitution really be one thing, or another, this much is certain--that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist." --Lysander Spooner, 1867
