Why not look for another couple who are looking for a woman? Most of these situations seem to involve a straight men and a bisexual women. Is there a problem having another man in the equation, who would (probably) be part of this extended relationship? This seems like an obvious and workable arrangement, so why isn't it more common? (Or is it, but I'm not seeing it mentioned much?)
My *guess* is that most couples seeking a unicorn also have a one penis policy. Deep down, they feel that homosexual relations between two women are less of a threat to the existing relationship. Also, in this type of scenario, the woman "joining" the relationship is usually younger, or less experienced or at a disadvantage financially. If she were married, then she would be more likely to assert herself, want to be considered an equal, and wouldn't stand for being treated as secondary.
This isn't the case all the time, obviously, but it is common enough to be a stereotype.