I wonder if people are just wired differently, and it's like trying to convince a straight man that he should actually be attracted to other men. Would explain why this topic has generated basically zero interesting discussion, which is unusual for this website.
I think you might be on to something. Maybe I'm an odd duck, but I can't possibly divide my romantic attention between multiple individuals without negatively affecting the quality of the relationship/s. I suspect anyone who claims to be able to doesn't experience love the same way.
You're right though, different people probably do have a very different experience of love. Some people say they do not experience romantic love at all, some people can't imagine romantically loving more than one person, others are sure that they're fully in love with multiple people at once.
It's impossible for me to say conclusively that someone is wrong when they're talking about their own experience of feelings.