and again, I'm not talking about bad parenting who fights all the time in front of kids - this is a different case. I'm talking about introducing a stranger to the formula that's already going through a bad time.
even in business, you open a store with a partner - then this partner goes out and becomes also a partner with your competitor. this is a conflict of interest and has destructive consequences for a business.
One spouse will be in a honeymoon kind of phase with her affair partner. the affair partner will go above and beyond to impress his cheater by bringing her kids to his side - a Dad or Mom will find themselves now competing to keep up just to keep their children close. the betrayed spouse not only has to go through the betrayal but now the feeling of lose the kids to the affair partner.
One colleague always showed her husband and two boys in the office, pictures all over the place, during conference calls she has the camera open and the kids and husband walk around and say hi, they looked happy. she always praised her husband. one month later, after they bought a house, she moved out to her AP. then her facebook account filled with photos of them going all over America, and taking pictures with her kids and AP. I was blown away, by her AP holding the kids as if they are his own etcc. he looks like he got money more than his husband.