Hi Linus,
I think most people are capable to love many people, if they allow themself. It is a matter of being open, free from what society norms are, and enjoying yourself with many. For some, the concept is do-able. However, in reality they can't for whatever reason (boundaries of safety, religious beliefs, kids, and so on). Sometimes one partner can handle it, and then when the reality comes into play, they decide that it is something that they cannot support or participate in. No matter how much talking is done, good and bad, it still kills it.
I agree, if you mention this to most people, you get looked at like you have 3 heads. Oy.
Have you watched the series "Big Love"? Good show.