I wonder if human nature might not come along with a finite number of more or less flexible normative templates for love-relationships. These templates would define the basic forms of love that are appropriate to us. An attempt to create new basic relationships, or to transcend the boundaries between these, would then be failures insofar as a relationship is defined normatively. Thus, two siblings might attempt to enter into a romantic relationship, and they might behave romantically together. But, nonetheless, they would not be having a romantic relationship, because the normative features of a romantic relationship would be objectively absent: thus, there would in fact be no obligation—but only a false belief in the obligation—to nurture the kind of intimacy that a romantic relationship calls for. We would say that the siblings are not in a valid romantic relationship, where we use "valid" in the technical sense in which we say that only an "invalid promise" (i.e., no promise at all, but only the appearance of one) results from coercion.
One way to make plausible such a story would be through a scepticism about the possibility of our creating new normative facts, other than by simply by making the antecedents of pre-existing conditional ones true.
Of course the difficulties then are epistemological: how do we know what all the templates are?