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Fifidog You don't have to physically cheat on your partner in person - an emotional affair can be enough to devastate a 20, 30 or 40 year marriage.
Just because he hasn't met her in person, it doesn't mean that it's a "harmless bit of fun"
You also have to consider how this has affected you emotionally. Could you potentially live with an open marriage or would you prefer a monogamous marriage between yourself and your husband??? Some people can make an open marriage work... but most people find that they can be a nightmare - especially if you're already feeling insecure in your marriage as it is.
That can slip into an abusive relationship as you start to fight against each other with one person trying to stop the bothersome behaviour and the other person not wanting to give it up.