If you ask most people how they’d feel if their partner had sex with or fell in love with someone else, the responses would be pretty negative: fear, anger, jealousy, rejection.
Ask a polyamorous person the same question, and they’re more likely to tell you they’d be thrilled.
It’s a concept called “compersion”…
Read more:
http://www.scientificamerican.com/article/new-sexual-revolution-polyamory/