The author suggests that good metamour relations are important to good communication and balance in a relationship dynamic. That communciation is a special and delicate matter with metamours. Ain't that the truth.
There is mention of a hidden threesome when one is dating a couple, but actually, there is always a hidden threesome or triad going on, even in a vee. The ends of the vee have their own unique and special relationship. Nurturing that is very important to the health of the over all dynamic I think.
Threesomes sexually kind of are a tell tale way of that relationship dynamic playing out in an intensely emotional and hyped up environment if I were to think about it. At least in my experience.