I definately think you have a right to your anger, but keep in mind that anger is often more damaging to you then to the object of your anger. It can eat you up inside and make it harder for you to trust people going forward. I would say work towards forgiveness in the way that Calenlass suggests. You may not need to voice that forgiveness, but having it will be an emotional victory for you because it will allow you to let go of the enormous violence of this event. Forgiveness isn't about saying "let's be friends again," it's about accepting that what has happened has happened and moving on. Far too often, people who have failed to forgive things from old relationships carry that anger with them into new ones. This can cause enormous harm in your life, much more so then any continuing anger will hold in the lives of the people that hurt you.
How to go about forgiving is going to be different for different people. OWW clearly needs to know why before he can forgive, and if this is true for you, you can go ahead and try to figure out why. If the details of why aren't necessary for you, and especially if they would hurt you more, then you don't need to go through that process. Keep in mind that an honest answer to the reasons why might be painful. Many people who cheat do so, in part, because they are dissatisfied with their current relationship. If you'd rather not know about that, then don't find out.
Trust, however, is a separate matter. I would suggest that you have absolutely no need to trust either of these people again. You can let go of the emotional anger, but hold on to the intellectual knowledge that these people are untrustworthy. What this often leads to is a cordial but not terribly close relationship. I have this with one of my exes. We can encounter each other, exchange pleasantries, talk about how our lives are going, but we can't get beyond that because I still don't trust her to be honest with me. I don't see anything wrong with this. I'm not angry with her. I've forgiven her. I understand why she betrayed me. But she betrayed me and I have no reason to believe she wouldn't do so again.