I'm in a triad (and I have a thread somewhere on the forum). We call ourselves a triad, rather than polygynus because there's no plan for marriage (and if there was, would be just as much between us girls as between e male and the two females), but we're also not a fidelitous group. [i will probably do some dating outside the relationship when time permits :-) ]
Ours started from friendship and practicality, but has definitely become about love as well.
But the key point with us is that there are currently three children involved, with the definite plan for more in the next few years, so I'm always interested in poly/triad parenting.
Our FMFTriad + 3 children + 2 pets = one crazy household