there are no words that I know of, that could accurately respond to the awful thing that you describe here. I really am very sorry.
There are many things you could do, in response to this, but I think your best response might be to do nothing at all. I don't mean in life in general, but in response to your soon to be X.
It sounds like to me, you should just keep walking on the high road. At this point, just be the best father and dad you can be for your children. Should a day in the future come, that you meet someone else, and want to have children, you can deal with that then. And I would say, be up front with anyone you meet in the future, when and if it becomes serious.
If you strike back at your now wife, you have to think about what that's going to do to your children. If she is really bragging about tricking you, well, I just don't even know what to say. That, is just a horrible horrible person. You might want to be glad to be rid of her knowing what you know here.