Infidelity happens.
We'd all like to think it doesn't. But it does.
That means, if you're aware of it. That your folks aren't the people you always hoped they were.
And that's the biggest let-down for any child. When they realize their parents aren't what you always thought they were !
Is the cheating partner a scumbag ?
Yes. Can't argue in their favour, really.
It's wrong to cheat and they know it !
But the crime is compounded by not only cheating, but GOING BACK HOME afterwards !!!
Made all the worse if the partner takes them back knowing what they do.