I find this an interesting question. As I've read here over the last many weeks, I've thought back to times in my life when I've been 'in love' with two guys at once. When I was married, I had a long-term crush on someone, but I still loved my husband very much. But I never once considered that it's any sort of orientation. I've always assumed that given the right person/people coming along, anyone and everyone can fall in love with two (or more) people at once. I still tend to think that, but it would be interesting to take a huge poll of the general public and find out how many people have never felt that way about two people at once. Maybe I'm unusual and don't know it. But if polyamorous simply means one is capable of falling in love with more than one person at a time, I believe the human race is generally polyamorous.
That said, I still believe it's always a choice to act on feelings, and a choice to enter a polyamorous lifestyle. My BF (the poly of the pair of us) feels the same way.