Two stories of personal experience
My BFF has been with her H for over 20 years. He has always had a low libido and when they were younger she thought she did as well. However as she got older she realised she was actually into BDSM and this was where her sexual response resided. Her H was unable and unwilling to do any remotely kinky stuff but gave her his blessing to explore the local kink scene - but he didn't want to hear any details.
They have now had this arrangement for around 10 years. BFF has a primary boyfriend and she lives with him and H 50/50 (they only live a street away from each other so it's logistically easy.) Her boyfriend has another girlfriend who stays with him a couple of nights per week. Both BFF and the primary boyfriend also have casual flings with up to 3 other people on the regular. H doesn't see anyone else - he is very involved in his hobby which takes up a couple of weekends a month and involves a lot of travel.
H has found the pressure is off since the relationship is open and he doesn't feel obliged to try to fulfill BFF's needs, or feel guilty that he isn't. He is much less stressed and more confident.
BFF, her H, her boyfriend and the other girlfriend all regularly socialise together, have xmas together, birthdays etc. I've been out to dinner with them and hung out with them in a group and there is no awkwardness at all, just a huge amount of love.
For me personally:
I have a FWB, and he's the only person I fuck on a non-transactional basis (I'm a sex worker.) He has a mild cuckold fetish and if I've had a particularly satisfying client, I'll take an intimate snap and message it to FWB with a suitable caption.
He enjoys group sex (I don't) and frequently goes to swingers clubs and parties and he'll tell me all about it. We both enjoy hearing about each others' exploits.
Last year I moved a significant distance so we have naturally not been able to meet up as often (used to meet once a week, now it's once every 6 weeks if that.) He recently met someone at a swinging club who he has really hit it off with and feelings are getting involved. He was very thoughtful in how he told me about it but I'll admit I did have a momentary twinge of "Huh, what am I, chopped liver?!" This passed fairly quickly though and I am genuinely now pleased to see him so happy. His new partner will continue to see her FWBs and FWB will continue to see me and attend parties etc.
So in the first instance I'd say this was a polyamorous reationship since there is a love relationship between most of the participants.
In my case I am fond of FWB and I would be sad if we parted ways completely, but I don't love him and would never consider a committed relationship with him (or anyone else for that matter.)
Hope this helps, and I'd recommend having a read of The Ethical Slut too.