I agree with many of the PP. I feel that it is near impossible to find a person who wants to be an addition to a couple, and if you do, you always have to take them into consideration. They may not have the same designs as you do, they may not be attracted to both of you, there are just way too many possible things to factor in. Don't get me wrong, I would SOO love a Unicorn to help us form a triad, but I am not going into this looking for one. I am just looking for someone to love, much like mono dating. If that someone happens to want to be with my spouse as well, BONUS!
Good luck with your Unicorn hunt!