Great point! I've told this to my wife many times in hopes of priming her for a conversation in the future. Also, you're right, the therapist didn't TELL me, it was more of a suggestion as to why I might be acting and feeling the way I do.Unlike some others, I am guessing your therapist didn't TELL you you were polyamorous. Maybe they introduced the concept to you and you found it resonated with your feelings.
Howeer, I don't believe some people are "hardwired" for polyamory, and some are "hardwired" for monogamy. From my experience and my many years of reading and talking to poly people and unhappy mono people, I believe it is in human nature, and in the animal kingdom in general, to be promiscuous. All humans at one point or another have been attracted to people other than their committed partner/spouse. But our culture established monogamy (for women at least) many millennia ago. Therefore, with the advent of reliable available birth control, more open relationships are now an option for humans.
Some people prefer the structure of monogamy and some just know no other option, or are not adventurous or brave enough to attempt open relationships. But more and more people are trying it and finding more fulfillment in their lives because of it.