It is interesting that some might fear marriage because they fear the passion will go away. As you point out, it's simply human for the passion to drop no matter what form of relationship two people have. That makes me think some people only intend to stick around as long as there's passion. Fair enough, so long as the people they're with know that and agree to it. I wonder what those vows would look like: "When we're done feeling passionate, let's plan to leave each other, so let's make it an easy route of departure by not getting married." So two people have to agree: "What are we doing here? Are we in this for all time, or do we just want the sort of arrangement that can easily be disposed of when we're just not feeling it anymore?" I guess what matters is that people agree on that.

A friend of mine used to say that the passion will inevitably decrease. So start out at 100% because it will drop over the years. You'd rather it go from 100 to 50 rather than start at, say, 60 and end up 0:)