If one of us did fall in love with someone else, we would figure out how that person now fits into all of our lives or whether we all want that person to fit into our lives. If my husband hated my girlfriend or vice versa, it simply wouldn't work. It does take a special combination of people. But more than anything it takes a great deal of understanding and love.
Now, should one of us decide we want to build a life with someone else separate from the rest, that would be a whole other ball of wax. We have definitely accepted that that could happen, although we certainly don't want it to. If it did, I think we would all be devastated, just as a couple in a traditional marriage would be if they decided to separate.
That's the thing—that could happen to anyone whether they are married or dating or living together or whatever. It has nothing to do with being open and everything to do with being human.
There are no guarantees in life. You could lock your partner up, I suppose. (But I can't imagine that you'd endear yourself to them that way.) You could follow them or have them followed. You could make them swear on their lives. But the truth is, people change, and relationships change, and wants and needs change.
That's not to say that some people can't stay together forever and ever just as happy as can be with no outside needs or desires. But not everyone operates that way.
Some argue that polyamory makes it more likely that one of us will want to be with someone else exclusively. But I think that quite the opposite is true.
Since there is no sense of being trapped in my household, there's no temptation of the forbidden. And since we are honest with one another there is no need to cheat or lie or sneak around.
We live with the reality that people are attracted to people other than their spouses or partners. But accepting that as a part of life doesn't mean we're more likely to leave each other. We may be more likely to follow our desires where they lead, but we're also more likely to see those desires clearly, and more likely to talk about them with each other, which allows us to understand them and make the best decisions for all of us.
Love is a risk. For me, it's a risk worth taking openly, honestly, and consciously. Open marriage is the level of risk that suits me and my partners. I think it's up to everyone to determine for themselves where their comfort level lies; only then can everyone flourish and grow and live out their own happily ever after.