Reading your letter prompted me to register and write to you. Your husband is cheating on you. Whether he has slept with these women (yet) doesn't matter, he is cheating on you. Men who are well taken care of in their marriage often don't want out of the marriage. They like their savings accounts whole, meals prepared, children cared for and lives intact, all things that would be too disrupted by divorce. They are hooked on the illusion of romance and think choosing married women to cheat with will allow them to avoid the pressure of the "affair woman" wanting more. But the married women he is cheating with will eventually want to be with your husband, for many reasons, one being that is just the way women are made. The romance usually last about 6 months, and then reality creeps in. If he continues to be addicted to romance there is no shortage of like minded people surfing the internet, thinking that is the solution to their problems. The irony is your husband ultimately wouldn't want to be with them because they are cheaters! But make no mistake, this is a powerful addiction, heady stuff, as strong as any heroin shot in an arm.
You are left with lack of respect from your husband, lack of respect for yourself, confusion, pain, fear...things that will only continue to errode any self respect you might have left. Don't treat someone better than they treat you.
It is hard to imagine that someone you love could betray and hurt you so much but believe me, it is going on. You can't reason, plead, pretend and think it will produce anything more than more pain and confusion in your life. Eventually you must make a decision to accept his cheating and stay, or leave. I suffer for you, having gone through this. If you leave, I promise one day you will hear a forgotten sound. The sound of your own laughter and happiness.