Do you have any objective factors to which you attribute that an "adult" would be drastically different than a sippy cup, the couch cushions, or the family dog?

Yes! Indeed I do. Though in fact the dog is an intermediate case -- an animate being with will, though not one who will interpret the experience in the context of human society.

An adult is another person, with desires, memories, a defined relationship to the person, values, etc. One who knows that you have mental models of him or her and knows you know she or has such models, etc. So any sexual pleasure will be perceived by the other, the causation will be wondered about, etc., motivations will be scrutinized.

You could as well ask if earnestly saying, "I want to be your best friend forever" is different when addressed to couch cushions, a sippy cup, a dog, or a person.