Why are there extremists?
People absorb information. It is part of human nature to learn. So, when someone acts as a leader, people listen. More and more people start to agree with the points made by the leader and they sway to that side of an issue. After all, it is much easier for people to listen to someone that claims to be a leader, than it is to take the time and effort to research the issues for themselves.
Sooner or later, it is inevitable that someone will act as a leader for the opposing side. Once again, people are eager to listen and some sway to the other side.
Before long, there is a competition between the two sides and each side starts to do more and more outrageous things to gain attention.
The strong opposing sides cause disarray within the general public. This disorder forces people to think carefully about the issues and for them to make their own decisions. Their beliefs may not necessarily fallow one extreme or the other, but usually lie somewhere in between.
The extremists help define the beliefs of the people that are between the extremes. Therefore, the extremes help to define the middle.
So when an extremist knocked down the doors at Stonewall, another extremist stood up in protest against the injustice.