“Tea bag is derogative.”
It is, how?
Assuming you’re asking seriously, the liberals—particularly those at MSNBC—love to call Tea Party participants “teabaggers.” For them, it is a reference to a once-obscure sexual act in which a man lowers his testicles into someone else’s mouth. According to wikipedia, it is usually another man. Most who use the term “teabaggers” as a perjorative know full well what they are saying, and this includes elected officials on up to the president (I’m pretty sure he’s used the term). The MSNBC hosts take particular glee in using it and usually say it with a knowing snicker. The kid with the sign here is playing into it in a “turnabout is fair play” kind of way.
"It is, how?"
As Anderson Cooper and his ilk in the MSM know very well, "tea bagging" is slang for a twisted sexual practice favored by the bath house crowd. (The same crowd that brought us "fisting.")