Social Incest

When groups of friends, much like family, all sleep with each other and/or date eachother in turns.

For example, social incest happens when Billy, Suzy, Drew, Ann and James are all good friends. Billy and Suzy start dating, then break up and Suzy starts dating Drew and Billy starts dating Ann... then Suzy and Drew break up and Suzy starts dating James and Drew starts sneaking around with Ann behind Billys back