The healthiest relationship I know of, is of a good friend of mine. The thing is.. they aren't married and don't plan on it. They have been together for 7 years, live together, and have a child. They take care of each other and keep it pushing. Now of course I don't know all of what happens behind closed doors, but anytime I've seen things possibly get heated, they handle it swiftly. It seems that it only gets there because of their family and other friends STILL pressuring them about marriage. It's pretty stupid.
It's very interesting to see how they maneuver. It's an actual level of mental maturity I don't see often. I also don't see them as a one off. I feel like it could be like this...but how things are now, it may be impossible for most to even fathom a long lasting relationship that doesn't include marriage. I'm happy for them.