The traditional image is that men cheat and that women don't. Everyone cheats, but for women to admit doing so would destroy a huge chunk of the image of them being "better" than men ("toxic masculinity", "patriarchy", "men are animals/a$$holes/only think with their dikk", etc). Also if a woman cheats, she will somehow, someway, make it the man's fault ('He's not paying me attention", "It's not like it used to be", "The other guy made me special again", etc). So she doesn't really gain much from an open relationship anyhow.