Why do couples allow each other to cheat on each other in 3somes?

Why do couples allow each other to cheat on each other in 3somes? I never fully understood why some couples want to have a 3some. That's pretty much saying, ';hey hon, want to end our relationship?'; Are people just out of their minds nowadays?Why do couples allow each other to cheat on each other in 3somes?
It never made any sense to me either. I would be sooo jealous to watch my husband have sex with another woman. And my husband gets mad when another man even checks me out ';with clothes on!'; That's the way it's supposed to be! I think everyone has different turn on's, but I never understood why cheating is fun or a turn on for some couples? Yes...I do think some people are crazy nowadays=)Why do couples allow each other to cheat on each other in 3somes?
yeah, everybody wants variety.
You know what.I couldnt agree more wit u.That has always been my theory.
Most couples don't do threesomes.

john masters