What is open marriage? It is a decision to love and appreciate your spouse even if they feel that they must love and spend time with some one else.
Does it mean 'sexual contact' with others? It might and it might not.
In the bible open marriage existed frequently! Abraham and Sarah...... Sarah was sent to live at another man's house..... Abraham conceived children with Hagar.....
Abigail and David....... Naboth was less of a man (a drunkard)..... David was a good man and Abigail noticed those qualities.
David and Jonathan..... David loved Jonathan...... but made no sexual contact..... only pure motives to support and encourage.
However, there are stories where infidelity is strictly wrong. Joseph and potiphar is an example of moral corruption in the heart. One person wants to use another person to gain selfishly. Another example is Hosea who dealt with an unfaithful spouse...... He continued his 'open marriage' despite these issues.