People who have open relationships don't advertise the fact or dash straight to AB and say "hey folks, we have an open relationship"
This is often kept to themselves.
One partner may know what is happening and turn a blind eye and vice versa....one could surely define that as an open marriage.
Yes I do know someone who has an "open marriage" but the excitement soon wears off as they get older and this couple have been married for 25years.
Before the "biddies" start crowing..........it is not my marriage that I have been describing.