Originally Posted by
IndiReloaded 
Mish, you may be right, but you are putting a value judgement on this situation that only the OP understands.
Perhaps YOU couldn't forgive her affair (I couldn't either) but he may be able to given the right conditions. Marriages do survive affairs.
If there wasn't a child involved I would suggest immediate break. But there is a child & therefore I recommend counselling. A good counsellor will be important to help maintain good relations & communication even if they split.
To the OP: I know its easy to be distracted by things like sex, but you need to be thinking bigger picture right now. Counselling is really in order & don't entangle yourself further with her until some ground rules are set. Which should, at minimum, require her to break all contact with this other guy if she wants to work at a reconcilation. Don't settle for less.
Also, you have a child to think of. Consider that, if you sleep w/her, you are also sleeping with whatever this other guy has potentially given her. Are you ready for that? Don't delude yourself, it happens a lot.
Check out that site & get counselling. Get your emotions under control, best thing you can do right now for everyone. Good luck.
But what can you do if the other party is just not interested? (In counseling or in working things out) It looks like she thinks she can do whatever she wants because she has control over OP, she can pull his strings and get her way (Probably have an affair and at the same time know that the door is always open for her to come back). In this situation no amount of counselling and good will help.
The only thing that will help is a firm action. A firm action that shows she's made her decision, now the course will change regardless of whether she likes it or not. She has no control now, no power to manipulate the situation to her will. I believe this is the only way there is a chance she will come back. Once she realizes she has no control she will rethink the entire situation. And if she doesn't then good riddance for both OP and his son (who's approaching adult years). If her mind is so dead set on this other guy, then they will do much better without the drama of a love triangle.
But you're right, this is just my opinion. It's not even about forgiveness for me, it's just practicality.