My current 3-year boyfriend, Mr. C, is an old high-school friend and we have a gaming/social group comprised of decade-long friends we meet with, frequently. This helps because we can be rather open with them and know they accept us. Our spouses also feel comfortable coming to the gatherings with us.
I met two other partners through OKC, but I'm not as close with them as I am with Mr. C, and neither I nor them have felt ready to share as much of ourselves as I can with DH or Mr C.
DH met one partner through work, and the relationship kind of fizzled out when he switched jobs. He met two new partners through OKC, and those kind of crashed and burned due to trust and compatibility issues. His current interest, S, is a mutual friend that we've had over a decade long friendship with, and although it's new, I have a feeling this relationship with them will last a long time.
Neither of us have really gone out to meet people as we're both really invested in our current partners and our son together, though. I can't figure out how that would work for us, really.
So I guess, for me and DH, we find it easier to form deep poly relationships with people we've already had long trusting friendships with. Dating sites like OKC are interesting, but a bit too scary for us.