"Discussions"

What is an appropriate way to have a mature discussion without people's feelings being hurt?

I always like to think that it is healthy for not everyone in a relationship to have the same opinion about the topic, and I enjoy hearing other people's sides. I also enjoy having the same respect back.

Recently, in a lot of my communication classes we have been discussing relationships and how they develop and fall, and we talk a lot about how hard it is to be in a relationship where everyone has the exact same qualities as the other. As my professor today said, "What the hell would you have discussions about? I'd get out of that if my wife and I were exactly alike"

I just was wondering at what level is a group of friends, or anyone in a relationship, mature enough to have a (let's rename it to) a debate-about a topic? And walk away afterwards still friends...no hard feelings?