I don't hear the word "unrealistic" in that way, YGirl. In any case, SeventhCrow seemed to me to be highly doubtful or skeptical of the possibility of a male-female couple finding a bi female who might possibly love both of them somewhat equally, and thus avoiding the pitfalls of possessiveness, jealousy ... or the "we two came first and so we two come first" sort of dynamic. Crow may be right that finding such a match is challenging, but to call it "unrealistic" sounds to me just a little too strong a term -- as if the couple was searching for a literal unicorn rather than the figurative one.
So much depends on the couple, to begin with. If they are like the pot of gold at the end of a rainbow... Wait! No... if they are truly golden people ... they deserve to be powerful attractive magnets for the equally rare other who may compliment their situation with bright and equal loving.