if it were that simple, we wouldn't be having this discussion.
The assumption is that if a person or group chooses to protest a funeral, they are choosing to be disruptive, and therefore the assembly is NOT peaceful. Therefore, it is not protected by the constitution.
Anyone has the right to say anything they want, but just like anything in life there are certain parameters. You don't have the right to say it anywhere you want.
The discussion is if it should continue to be legal, not if it is legal. If it weren't legal, surely members of a Baptist church wouldn't commit such an act, at least without being arrested for their illegal actions.