It's not as complicated as it sounds. With a very few times exceptions, I've only been sexually involved with one guy at a time. However, I tend to love multiple people and retain affection for old boyfriends, etc. There is a lurking potential in the background. And sometimes we are very physically affectionate, though we are not attached "that way." But I consider myself a polyamorist at a very cellular level.
This is especially true if you look at my relationships with the other women guys I've been involved with are also involved with at some level. The best relationships have been where the other women are also good friends of mine. The ones that distinctly don't work for me are the ones where the guy tries to keep his women separate and separated. And when I'm out of a physical relationship with someone, a lot of the time, I find I get along splendidly with the current partner.
Hey, if you can have serial monogamy, why not serial polyamory? Shrug. Just me. There are so many variations to how we humans relate to each other.