Nycindie: The word possessive doesn't have inherent meaning, and just because you can frame things to sound bad doesn't mean that they are.
Many monogamous people might feel short-changed if their partner lavished attention on a platonic friend. This is for the simple reason that the choice to be monogamous is most often based on the choice to be in a partnership where you use your available resources (time, energy, attention, etc) mostly with one person. If that's your definition of possessive, that's fine, but it's pretty convenient to define something that you don't involve yourself in using a word that has culturally negative connotations in all relationships. There's nothing inherently negative about that.
A lover who needs his wife's attention is interfering with a sharing of resources considered appropriate in monogamous relationships. It's not that they're over-utilizing your property. The sharing of available resources is something that should be laid out and found agreeable at the beginning of any relationship, which it unfortunately does not seem to have been here. For a partner to join a group of friends that doesn't include you again would depend on the partnership you BOTH desire, not childish feelings of desperation.
Furthermore a lover without question demands more energy and attention than a friend. They also demand "time sharing" in a way that would not be considered appropriate to a monogamous friendship (because of the way most monogamous relationships lay out sharing of resources). It has nothing to do with maintaining your court and throne.
Quite frankly, I find it disgusting that someone who might be monogamous drifts onto a polyamorous forum struggling with something their wife wants, and you chastise them. Anyone in his position is in no position to argue what I'm arguing. They'd be most likely to feel bad about themselves for being immature and unenlightened rather than understanding that perhaps this simply isn't a style of relationship they personally want. You're setting him up to go back to being miserable by setting him up to see that his needs (which may be intrinsic to a style of relationship he needs but is not in) are inferior to his wife's more mature needs.
I hope you would find it appropriate if a potentially polyamorous person struggling in a monogamous relationship was asked on a relationship forum, "Are you unwilling to commit? Do you fear being with one person? Are you afraid of devoting most of your time and energy to a partner you cherish?" That's tantamount to what you just did.
Turtleheart: Unfortunately the issue of time/focus is what is the central change between different styles of relationships. As I noted above, the available resources to give in a relationship are time, energy, attention, etc. Monogamous partners are those who decide that they individually want to give most of those resources to one person and want to receive the same amount in return.
If what you want is the level of time/focus present in a monogamous relationship then there is no way to resolve the time/focus issue. If there's some level of time/focus in the present polyamorous setup that would work for you then it can be resolved. Unfortunately, as I mentioned before, this is really a decision you need to make on your own. You deciding one thing is right for you doesn't mean it's what will end up being right for your wife or J.
You have to be willing to look at what's going to make you happy in a relationship while still respecting the views of others you may be involved with without adhering to them. You can only really figure that out for yourself as an individual, and time/focus is the central issue of that.