Statistics tell us over half of us have already went through it, or will. I am NOT a big beliver in couseling as far as "saveing" the marrage goes. That is assumeing she did cheat, and you are sure of it. You mentioned a "old flame". Was that before or after you got together? If after, that is saying you already been through it and had tried to forgive her. If it was before you, it dont change anything anyway. Was just curious.
You do need to solve any question of cheating in your own mind 100%. If so, you need damage control to protect yourself. Everything I am writeing is assumeing you are the innocent wronged party here, and assumeing it is all one sided. I/we here dont really know that one way or the other.
In my single days on at least two occasions I dated fine ladys that had kids and wanted to marry me, but I wasnt up to takeing them on. I quit the affairs because of that, right or wrong. (If I wasnt up to raiseing their kids I had no business in dateing them to start with, but I did.) I did get married again and my wife has kids, but they were grown and out on their own.
I suppose laws are different from state to state. My divorice was in california. The fact is, unless you are very ritch, the courts in that state at that time didnt care who done what to who. It was all about money. My lawyer litteraly had to pinch my ear to get that through to me!
Now one side thought here. This may be for other readers. Why is it that some of us would be so quick to find out if the kid/kids are really ours or not? I must assume it would be to hopefully get out of child support if it comes to divorice. Surprise! Most times it wont really get you out. Especialy, if the woman wont crack and rat off the blood father. I never heard of one that would in those circumstances. Someone is going to support that kid, and odds are big it`s you! Yet many do marry another woman with kids and take them on. Maybe the same guy that would go the DNA route with the first wife?
My ex wife cheated on me. Seemed I was the last to know. At the time I was in love with her and would have trusted her with my soul. It tore me a new one. I wasnt worth shooting for a few years. In between fights she totaly admitted her new affair had nothing to do with me.
Where I went wrong was I initaly tried to save the marrage. That just made me more miserable. If this is as it sounds, end it now, protect yourself the best you can, and leave the kids out of it and move on! Yup, far easier said than done, but work with that in mind the best you can.