The difference between a gang-bang and an orgy.
In a gang-bang, one sex must out-number the other by a margin of at least 5 to 1. Any less, it's an orgy. Don't ask me what happens when it's all one sex.
~ + ~
And on a totally unrelated note (I swear!), would there be anyone out there willing to beta a Torchwood fic for me, if I crack on and get it finished and final polish applied tonight? It's got to be posted Sunday... *looks hopeful/pleading*