The bonding of a male, female, and pet in a bond. The pet is entitled to all benefits including sex and inheritance. Just like marriage you may divorce, but you have to divorce both spouses not just one or the other. Once a union triangle is made animal abuse becomes domestic violence and you and your human spouse will get financial benefits from the bank if the pet is to die of natural causes. If it is done with a cat they are joined in holy meow-trimony. Only and all land mammals can be accepted into a union triangle.
John and Mary loved each other and their pet cat Fluffy, so they decided to join all three together in a union triangle.