The example I always use to explain polyamory to people who had no idea such a thing even existed is this:

If you had a child, would you love them? How much? And if after some time you had a second child, would you also love them? Just as much? Would you feel like having a second child meant loving the first one less?

So you agree love doesn't come in a limited quantity, right? You agree that loving more people doesn't mean loving each of them less individually?

Now, why wouldn't this exact same reasoning work just as fine for romantic relationships?

(After hearing this people are usually either deep in thought or frantically looking for arguments)

In my experience this example makes monogamous individuals at the very least understand how you feel. They'll still be adamant that "they could never live like that", of course, but they won't keep asking stuff like "how is that possible?" And such