The analogy goes like this:
Imagine you have a refrigerator. One day, it stops working - you come home to find your ice-cream melted, your milk warm and smelly, and your vegetables wilted.The last option is patently ridiculous; and yet it is the one most people in the poly community seem to choose. That is, when someone feels jealousy at their partner doing a particular thing, they make a rule against the thing rather than trying to find the cause of the jealousy.
You have three options at the moment: you can figure out how to fix the refrigerator; you can throw it away and buy a new one; or, you can decide to keep it as it is but never buy any foods that need to be frozen or refrigerated.
The article I linked to above goes into LOTS of detail explaining it, and I would highly recommend it to anyone who hasn't already read it and is interested in polyamory or open relationships.
I had an experience the other day that made me think back to the refrigerator analogy:
Miss Prism was joking to a female friend of hers, asking her if she wanted to have a threesome with herself and my beloved Mr Oldman. Watching this happen in front of me made me seethe with jealousy - I wanted to be part of such a threesome, thankyouverymuch! I wanted to provide Mr Oldman with an outlet for that stock straight male fantasy!
It got to the point where I was going to pull them both aside and tell them I wouldn't be comfortable with Mr Oldman having a threesome unless I got to be a part of it, even though I knew this proposition was a joke and the chances of the threesome happening were almost nil.
Then I thought about the refrigerator. Clearly, this hypothetical refrigerator of ours was broken and I was about to propose he stop buying frozen foods. Really, I should try and fix it instead.
I thought about Mr Oldman. I imagined the joy on his face if this hypothetical threesome were to occur, and immediately felt a little bit of compersion at the idea. I imagined how wonderful that day would be and who would I be to take that away from him!
Then I thought about why I was feeling jealousy - it's because I wanted to do something nice for my partner. I wanted to make him happy. And him having a threesome with two other women wouldn't infringe on that; in fact, my making a rule against it would have been the exact opposite of what I really wanted!
So, Miss Prism - if you can get that threesome for Mr Oldman.... go for it :)