Erosa, thank you for the post! I've come to the realization that there are others like me... (Yeah, that was a half-hearted quote from "The Last Unicorn.")
I have at various times been open to the idea of dating an established, loving, healthy couple, and have come very close to it. The first time it was a couple that invited me over for drinks and to "see where things would go," but things never went anywhere. None of us had enough guts to get anything going so we just sat around, listened to great music, talked about life and love, and I still count them among my good friends. If things had gone sexual, I doubt much would have changed. They're very sweet, decent people.
Another time I was approached by a physics professor and his Pagan wife. They were poly and the husband wanted to date me, but the wife was incredibly supportive... She even offered to give me one of her vegan cookbooks! They lived very far away and I soon found myself dating my current and we decided to be exclusive, so I never did go out with the professor, but I still think that might have been a good situation.
I think it all depends upon perspective. If everyone involved is clear on their expectations, I think it might be an okay situation for the unicorn. There have been times when I've admired both members of a couple and I've wanted very much to be with both of them, so I think a triad could be a beautiful thing with the right people involved. I'd wager so say someone out there is making it work, although they might be in a very slight minority!
I've been on both sides of this, though... I've also been in monogamous relationships where I thought it might be nice to include another person, but haven't for fear of jealousy and other such reasons. Still, if my partner and I hit it off with someone that was able to love both of us, was attracted to both of us, and wouldn't try to break us up or play mind games, (there's the rub,) I don't rule out the possibility of forming a triad. It's not a thing I think I'd like to actively seek out (that hasn't worked at all in the past) but if it happens, it happens.