Hello and welcome to the Forums!
It certainly sounds as though you and your partners and metamours are in a position to explore yourselves and your relationships in a healthy and positive way. Your metamour (D) may need lots of reassurance from your paramour (S) - it sounds as though he is willing to give it a go and with your and S's patience and compassion as he weathers the paradigm shift from monogamy to polyamory.
I hope that you find the support you are seeking by joining us here!
Yes, loving more than one IS different than having multiple partners for sex only - a whole nother level. A learning curve for each person - one I am exploring in a new way currently (one of my primary partners, as opposed to FWBs, being open to developing new significant relationships outside of our established Vee).
Good Luck and Welcome (again)!
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe