So poly, then, would seem to be the choice between the benefits of maintaining a more full individual freedom in terms of sexuality and romance in order to experience the potential growth caused by the variety of partners or the benefits of complete devotion of time and energy to another individual at the expense of other potentials in order to fully explore that individual and everything they have to offer you. Both are alluring, and I think that they are mutually exclusive (not to imply that they are black and white...shades of gray entirely possible). I also think that they are equally mature when neither of them is taken to be a fairy tale ideal.
I don't have time to respond deeply so I'm just chiming in so I don't lose the thread.
But I agree in this quickie sense...
1) Poly and mono are both valid rship structures
2) Yes, there can be immature ways to do mono and immature ways to do poly. Yes there can be mature ways to do mono and mature wats to do poly.
Yes, I do not think mono is not just automatically the "immature" form of poly.
3) When you go mono, you have the possibility of going deep. When you go poly you have the possibility of going wide. Neither is guaranteed, of course because people are not things and the other partner's have their own wants, needs and limits. They may be mono but not WANT to let you know them deep. YKWIM? Ditto poly -- how far wide? Everyone has a polysaturation point, and I view this both in terms of number of partners and how deep you can even get to go with them because of that number.
Now I have to run, but I'll enjoy mulling all this over some more later. Thanks for the food for thought!