Emotional Energy and Time Restraints
I am of the opinion that it is not possible for ONE person to fulfill all of another person's wants/needs (and it is exhausting to try). Many people have outside friends and family that fulfill some of those needs. Polyamorists have other partners that also fulfill some of those needs.
For myself, I am a fairly extreme introvert but my job requires me to imitate the extrovert for many hours a day (meet a client, charm them into telling me their personal problems, fix their problems...move to next client - every 20 minutes all day long). I have very little emotional energy at the end of the day for superficial social interactions, and very little time to recharge my "people-interaction" battery.
How is this relevant? (you may ask) Pre-Dude I had a husband, a few very close friends, and a few more close friends that I could interact with without drawing down my emotional energy meter. After Dude came into our lives I found that my tolerance for spending time with our close but not very close friends did diminish (but my need for my time with my husband and closest friends did not). SO, having another partner did affect the energy and time I had available for people who did not have such an integral role in my life but not for the most important people in my life.
(Incidentally, I see the same thing happen to people when they have a baby - close friends and family remain but there is no longer time/energy for more peripheral friendships to remain at the same level.)
This is just what I have observed in my own life. YMMV.
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