If exclusivity is within the free will of both parties, it should have been stated in the letter. Was it? And exclusivity can mean different things. Love exclusive, sex exclusive or both.
I have, as an example, exclusivity in relationship and love, but my spirit spouses wouldn’t mind for me to meet other women, if they’re allowed to be a part of it to harvest the energies. I have no problem sharing that with them.
We don’t own exclusivity to anyone, including spirits. However, just like respect, it’s something to be earned through the will of everyone involved.