Here's how I rationalize the loving of more than one person at a time in a way that most people I talk to seem to grasp. Take this with a grain of salt, though, as I'm not official "out" yet and not officially a part of a polyamorous relationship yet. I'm still very new to all of this, but this is how I explained it to my wife, which helped her to understand why I associate myself as being polyamorous.
If you have two children, do you love one more than the other? Or do you love them equally for different reasons? Does the birth of the second child diminish your love of the first child? I certainly hope not.
Furthermore, let's say you have two best friends. You certainly love those two best friends and very likely for different reasons. Does the love you have for one best friend diminish the love you have for the other? Are you merely hanging out with both best friends so that you can eventually drop one and choose the "best" best friend? I would imagine not.
The only difference between the love you have for a best friend and the love you have for your life partner is that sexual intimacy is involved with your life partner and not your best friend. That's literally the ONLY difference (beyond the early infatuation). Love is not based on sexual intimacy, so it shouldn't be hard to grasp why and how you can love two people. All you're doing in polyamory is introducing sexual intimacy with a second partner.
Society is way too hung up on sex.