The rule, the boundary, that applies in these cases is that sexual interaction between professionals and those who hire them is likely to damage the integrity of the professional relationship and is therefore not okay. Sexual misconduct is an extreme example of breaking faith with those whom one serves. Clergy hold a position of leadership, and therefore of power, in the faith community Good professional boundaries define the limits of that power. We entrust clergy with power so that they can use it toward certain specific ends, and we rightly expect them to look out for our interests in the process. Boundaries are the rules that define the right and wrong uses of clerical power.
Boundaries are also the dividing lines between people, the limits that define selfhood and identity.
Psychotherapists use the term to describe the degree of psychological individuation versus enmeshment between individuals. My son broke his collarbone several months ago. In the emergency room, the doctor pushed and prodded to see where it hurt, which I am told is a good way to locate and gauge the extent of the fracture. I, of course, watched anxiously and when he hit the sore place, I flinched. It was a classic moment of boundary confusion, and it irritated my son, who rolled his eyes and insisted that it didn't really hurt that much anyway.