My gf's bf M is partnered with a woman, L, and they've worked out a once a week overnight schedule. My gf goes to their place on Friday night, and L is usually there. Once in a while, she goes out for the evening. One time she was out of town, which of course gave them more freedom.
They are also flexible, in case special occasions arise with them, or with my gf and me.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years