More importantly, other commentors here have already admitted to flagging this entry. The way flagging exists now rewards one-sideism and partisan behaviour - all it takes is a relatively small group of discontented people to take down a story that is otherwise interesting to the vast majority of posters. A counter-flag option would balance things.
That's not accurate, because if a story is interesting to the vast majority of users, it will get lots of upvotes—and lots of upvotes is enough to defeat a small number of flags. In that sense, we already have the counter-flag option you're arguing for.