There really is a lot of insecurity creeping around on these forums...
Alright, I'll give you my take on it, as a polyamorous guy who has been both in open and monogamous relationships, and with single and multiple people involved, I think I have a decent shot at explaining some of you what it actually entails and why it really isn't so "weird" to consider.
Ask yourself what you are looking for in your partner. I think most will answer "I am looking for someone to complete myself". Now consider how unfair that actually is towards your partner, as nobody is perfect, and happiness should not depend on someone being in your life or not.
Having multiple partners is admitting to yourself that multiple people can offer things in a relationship that are unique, worthwhile and worth keeping. This is unfortunately the essence of a lot of cheaters in monogamous relationships too. They look for things that their partner cannot offer them in others, but keep it a secret rather then talk about it in the open. They have a feeling they have to choose, and obviously have little faith in their partner being understanding of it. In other words, insecurity.
Having multiple partners is admitting to yourself that multiple people can offer things in a relationship that are unique, worthwhile and worth keeping. This is unfortunately the essence of a lot of cheaters in monogamous relationships too. They look for things that their partner cannot offer them in others, but keep it a secret rather then talk about it in the open. They have a feeling they have to choose, and obviously have little faith in their partner being understanding of it. In other words, insecurity.
Now for lasting relationships. I understand many feel like open relationships cannot last. And I ask myself why. It is just like any other relationship, you keep honest and stay open, and talk about things. Of course since there are multiple people involved, that means that the odds of not being compatible with one of them is a little bigger as when you're with one person solely. However a good friend of mine has been in an open relationship since he was 18 and he is now 33, just goes to show that if all those involved want to make something work, they will make it work.
Questions? shoot, I will try to answer