My husband has a non-sexual partner (she hates g/f or "other wife" title), she has adopted the title of "sister" and in many ways it fits. She is married, but it works amazingly well for all of us (now). Their emotional relationship gained strength as his and mine was pretty rocky and it did cause issues for me on different levels over the years, but usually only when things were bad between us. If they came to me tomorrow wanting to add sex, I would be ok with it (NOW), but for so many reasons I never see that happening, but you never know. We aren't a triad, but an "emotional Vee"
She and I just went out to dinner last night, just us girls.