Outstanding thread!
To us (everything is open for perception) the overarching condition is called an 'open relationship'
From that, we divide down into specific areas of interest.
Swingers - Married folks who tend to play with other married folks.
Cuckold/Hotwife - A Cuckold (in today's evolving context) is a male whose wife (Hotwife) sleeps with other men and humiliates her husband in the process (i.e. penis cages, having the husband serve both the wife and her partner, degrading talk, etc). Remember, a 'Cuckold' originally was a man whose wife had an affair without his knowledge. Those that knew referred to him as 'The Cuckold'. In fact, being accused of being a Cuckold in public led to many duels and subsequent bloodshed!
Stag/Vixen - A Stag is a guy whose wife (the Vixen) enjoys other men. The difference between a Stag and Cuckold is that the Stag is never humiliated and fully supports, and assists, his wife with her interest. He's fully involved and enjoys that involvement. Without a doubt, my husband and I are in this kind of relationship.
There are of course variations - BBC Only, Other Married Men Only, Only Single Men, Only Asian Men - the list and variations are endless.
Over time, (in the 50's through the 70s) Swinging was very prevalent. Especially after the pill became more accepted and safer. Then came AIDS and things slowed down a lot. It became to pick up again in the 90s and that's where the divisions started becoming apparent. First it was the Cuckold/Hotwife dynamic and this further divided into the Stag/Vixen sub-group that we see evolving today.
Whatever you do, don't fall into the trap that relationships and lifestyles are static. They are as fluid as the evolution of language and euphemisms (absolutely no one says 'Geepers' or "Golly Gee" anymore). In five, ten or fifteen years, we may see another dynamic start to evolve; perhaps the reverse of the Cuckold/Hotwife or Stag/Vixen relationships we see today. Maybe the women will be content to watch, or be humiliated while the male has sexual fun with other women. Or maybe even the traditional concept of marriage will evolve into something vastly different. Who knows?
Remember, the only constant is change.
Again, awesome thread - enjoyed contributing to it!