I don't just let him go off and have sex with people... mind you. This chick is an exception because she's a dear friend of mine and he and I have messed around with her before. It didn't feel unnatural.
We like being together with other people... we don't wanter odd. It's a shared experience. And it's been a long time since we've had a relationship with a girl or couple where separate stuff occasionally happened just because we all felt datey and comfy.