I'm sure it is a matter of thinking too. You don't just feel that way for a short time. It took talking to psychologist for them to say I belong on the Eastern side of the world where they practice two husbands. I don't take my sexual life as casual, but I certainly know of what I feel.
I think when you end up with the two women scenario, you end up with people who come and go and never end up with a long term relationship... but in most cases, the opposite, the women are thinkers and weigh what the right people would be to put in the mix
because when everyone's needs are met, it's not much different and twice as happy