I doubt they got into a fist fight over their opinions, the fist fight erupted over their delivery of their opinions and the lack of respect each gave the others' opinion.

Holding different opinions is natural to everyone. It is the treatment of how one listens, your body language while listening, the amount of time given after one stops speaking before a counterpoint is given. Fights erupt when people do not listen, and of course when one side can't articulate their position and the conversation devolves from mature.

Two respectful mature adults can talk about any painful subject. If you can't, that's an indication of a need to mature, either yourself or your attitude towards others.