Here’s a brief excerpt:
When it comes right down to it, perhaps the biggest unspoken reason people have for objecting to (or being offended by) polyamory is fear. It’s common for monogamous people to fear that a partner might leave them for a polyamorous relationship (or might demand opening up the existing relationship) if polyamory becomes normalized. But if your partner would actually leave you, or demand that you open up your relationship against your wishes, then you obviously aren’t on the same page. There are tons of people out there (I’d wager a large majority of people) who want mostly or completely monogamous relationships – and they should find, date and marry other people who want the same thing.
JulietEcho also wrote a defense of polyamory on this site several months ago.
If you know nothing about polyamory other than the stereotypes you may have heard, you owe it to yourself to read these pieces. I know I’ve learned a lot by reading them.