Step 1 When your girl friend has an ex boyfriend there most likely will be heartache in the relationship, if she tries to pursue a "platonic" relationship with them.
It doesn't matter if your girlfriend doesn't want to be intimate with her ex, it still could happen, because women are governed by their emotions.
So provided the right emotional state, she could cheat.
This isn't her fault it is just social human behavior.
Step2 When faced with the situation of your girlfriend wanting to continue a relationship with her ex, you must not give any reaction.
No anger, sadness or anything.
Just say that you are not going to continue this relationship with her if she, wants to continue to see her ex.
Any emotional reaction from you suggest that he is more alpha because you are now reacting to his frame.
You only emotionally react to people who are higher value than you are.
If she were to have lunch with Steve Earkl you wouldn't react to this, because you perceive yourself as having more sexual value than he does.
The opposite is true when she is going to lunch with someone who looks and acts like Denzel Washington.
So if you give a emotional reaction you are showing that you perceive the person to being higher value than you.
This is shown true when an abusive male cuts all contacts of his girlfriend.
The abusive male obviously isn't very confident, because it takes some real self worth issues in order to abuse a woman and he is cutting her contacts from the rest of the world, which means he perceives the rest of the world to be more valuable than he is and he doesn't her to see that.
Step3 So when faced with a girlfriend who desires to be in a platonic relationship with her ex, just say no and give no reaction.
Otherwise you project that he is higher value, and thus she has to find out.