I've only actually spent a significant amount of time around a protest march/meeting on ONE occasion.
That was at the Mall in D.C., during a war-protester march and a veteran's Gathering of Eagles counter-protest, where I was an Event Marshal and tasked with trying to keep the peace in coordination with the DC Park Police (a very professional organization that earned my deepest respect that day, btw).
It was quite an education.
On both sides, there were those who conducted themselves with dignity and restraint, and those who seemed flat damn determined to provoke someone into violence with extreme verbage and actions. Both sides seemed determined to drown each other out much of the time. There were an estimated 30,000 present, and it was a chaotic madhouse.
This caused me to conclude that protests and counter-protests are NOT places where one may expect polite and civil debate, or even much reasonable behavior, out of either side. They seem to feed off each other and get more extreme as the clash goes on.
To get to my point... where there have been instances of pro and anti SSM protests/counterprotests, I've no doubt there were uncivil acts on both sides, perhaps including people trying to drown each other out.
However I would not attribute such behavior as a general norm for either side, since such encounters are so chaotic and uncivil by nature.