Numbers exist. My computer exists. I can touch my computer, but not a number. Prima facie, their must be at least two kinds of existence. I go with physical and explanatory.
A cave exists in both ways. There is the physical existence of various elementary particles at various points in space at a given time. And there is the explanation that it is a cave.
If Jack and Jill go to the park, there is the physical movement of their atoms to the park, and the explanation that friends are having a picnic. The physical description doesn't even know the location titled "park" is a public place with grass and trees. It just has coordinates in space.
Anyway, what this entry is really about is Relationship Theory:
Premise: "Relationship" is an explanatory term. It does not describe a physical event.
Obligations are explanatory. They also cannot be deduced from pure logic (because they depend on things in the real world). What they are, is when certain events happen, what is right to do changes; obligations are alternations in the moral landscape. For example, agreeing to meet David at the park, changes the moral landscape by making showing up at the park the right thing to do in scenarios where it otherwise would not have been.
Premise 2: To be true, explanations of why an obligation exists must, at least indirectly, refer to something physical.
Conclusion: Relationships, in and of themselves, do not create obligations.