Should a one time open marriage end with divorce?

My spouse and I have been married for about 8yrs now. We have young children. 3 months ago or so, he decided to form an attachment to a married woman who happens to be a longtime friend. She in turn became attached as well! Rather than put selfish feelings n desires aside, both of them continued this relationship with their spouses aware of their attachment. Now here is the kicker! My friend's husband and myself became closer in time and developed an emotional attachment as well. Emotional led to physical, you get the picture. Now that jealousy and resentment have reared their ugly heads, where do we go from here? Continue trying to make the friendship work? Cut eachother off completely? There are so many unanswered questions stemming from our past actions. Lately the big pink elephant in the room seems to be divorce! Is divorce really the only solution?