I disagree. Even as an adult I would be distraught if one of my parents decided to cheat on the other. Your children being adults doesn't mean you no longer have a responsibility towards them.
If this is your attitude then you should just never get married in the first place. At the very least you owe it to a partner that expects a real commitment.
For one, there's no custody case. There's often also no child support (some jurisdictions extend it to college students) to speak of.
There's really mostly the sorrow of the parent who was cheated on.