My answer for couplehood versus polyamory: I can tell Kristy or anyone else that she is my bestest, bestest, bestest friend in the whole world, and there isn’t one person in that entire world whom I can offend by saying that.
That is the explanation which I have sought for years and which came to me unconsciously during my walk. My previous defenses of couplehood were in terms of its simplicity, but here I have found an example of what one loses by increasing the complication.
My answer for couplehood versus singlehood is that I then bring some of the most intense aspects of life into the realm of this bestest, bestest friendship.
I do not wish to pronounce couplehood the best choice for everyone. I certainly know people for whom singlehood is the best option, because they cannot bear to have relationships of even moderate intensity, and regarding others I have no opinion one way or the other which of the three options is best for them. After all, it seems pretty obvious that polyamory offers pleasures that couplehood cannot provide, and which may be sorely missed. The rewards of couplehood are notably bittersweet.