it is because that you are same like your wife mean have girl friend and doing may be something wrong with her (may be), i don't know which country and which society do you belong,
first think on yourself that if you are alone with any girl then will you can stop yourself to do with her something and she too?
Reason is that first it totally wrong to making boy friends or the girlfriends, if you do this meant that you just want an illegal relationship with them, like wild soul wishes "porn".
if your society allow that is not wrong then think how to stop illegal relation and doing something wrong between boys and girls, so if you think that is wrong then leave her for ever or forgive her once more.
first time mistake is called mistake but same mistake if repeated its not called a mistake, if you forgive her 1 time then trust on her if mistake is repeated then it completely show you that she is not sincere with you,
please first time trust.
My Best wishes with you dear friend.