It was asked in the FetLife Poly Fidelity group today why people decided to be in a closed poly relationship rather than an open one. I wanted to answer this but my response was a little broad so I decided to post it here.
I suppose my first reason is a health one.
Keeping the number of partners you're intimate with low keeps the chance of contracting a sexually transmitted disease/infection low.
The second reason is more of a spiritual one.
When you have sex with a person, your sacral chakras connect. Those people with a strong connection with the sacral chakra can connect to your other chakras and effect the way you view yourself greatly.
For example Billy and Julie haven't had sex. Billy tells Julie she's fat and she ignores him. Billy and Julie have sex and then Billy tells her she's fat and it hits her at her core, effecting how she sees herself.
It could happen the other way as well. Billy could tell Julie she's the smartest person he's ever met and it boosts her self image.
If Julie only ever had sex with Billy, she would only have 2 people connected to her sacral chakra, Billy and her mother (by giving birth it creates that connection). So no two people could have as strong of an effect on Julie than those two.
My third reason for keeping relationships closed is due to my own personality.
I am a romantic and I fall in love sort of easily. I don't fall in love with many people but...I tend to fall in love with a few people quickly and strongly. This can cause some issues with my emotional health, as I'm sure you, my readers, can understand.
Having sex with someone I'm in love with only strengthens this and, if I were to be with someone that did not love me back, well...it's happened and it crushed me for about 3 months straight...I don't wanna do that again.
Closed poly relationships allow for security in health, a close spiritual connection, and a chance at emotional happiness.
The benefits of a poly relationship, especially a closed one are numerous. There have been articles and even a few books written on the financial benefits, the benefits in raising children with more than 2 people to help, and so on. Here are just 3 more of my own benefits added to the mix ^_^