Imagine being in a non-married relationship and taking a mortgage for a house you'll both live in and potentially both pay. You'll still need to have these uncomfortable discussions, probably upfront and not when it goes south. Same if you have a kid, if one quits their job to take care of that kid (or take care of the other if needed).
A marriage will package a defined set of rules to apply to these situations, where not being married will force a lot of case-by-case examination, with probably one end of the relation getting shafted. Some countries (Japan is one, there must be others) have a "not married but could as well be" status for these kind of situations.
What I'm saying is, being in a marriage or not is akin to having a contract or not. It doesn't change what you're supposed to be doing, it will only help to frame the discussion in the dire times. By the same token, breaking up a long lasting relationship shouldn't be about whether the paperwork is a PITA or not, not being in a marriage doesn't make it OK to just screw the other side for instance, hopefully you'll still have the uncomfortable discussions either way.