There’s nothing wrong with having an open marriage.
As long as both people in the marriage know that they’re in an open marriage and are okay with it.
My ex had a side chick for over a decade. When he decided to pursue Mollie, he didn’t first talk to me about it and suggest that we open our relationship. He just courted her and kept me in the dark.
I wouldn’t have consented to opening the relationship — but I would have thanked him for his honesty and offered to remain a loyal and supportive friend. (But not a girlfriend.)
Instead, I caught him cheating and threw him out.
The women in this group who had ex with their exes are all doing it without the knowledge of their exes’s new partner.
It’s not an open marriage if only one member of the couple knows that it’s open. Then it’s just cheating.