I think strongspirit is right. He may not react at all to your letter. He is already engaging in morally bankrupt behavior without regard to you or your wedding vows. When I use to approach my stbx husband about anything, he would often respond by saying, "here goes one of your lectures!" Another one of his responses was "I just wont talk at all!" It is fundamental to have the willingness to communicate with your spouse in a respectful manner. Infidelity is a very large waving red flag that in my personal opinion is a deal breaker. You can't teach accountability or insight. In response to your letter, if it makes you feel better then you should give your husband the letter, but I certainly wouldn't have the expectation that your husband is going to have an emotional epiphany. Put your energy into what you can do for yourself. The best revenge so to speak is for you to move on and live a happy, fulfilling life. The chances are your husband & mine will probably eventually cheat on the women they are with. Good riddens!