How members of the family will react will depend on the dynamics of the family, how the affair resulted, its economic effect on the family's finances. Because we live in a monogamous society, how discretely this activity is conducted will also impact the reaction to it, and how it was uncovered. If the children are young, chances are they will not confront the parent's action, but say if the children are grown teens or young adults, they may directly approach the parent for an explanation. If for example the mother as an affair partly because of financial assistance due to the father being unable to provide for the family, the rest of the family may overlook the affair. If the father has a fling for fun or to feed his ego, the family may get disgusted by his actions and arguments may flare. It can be quite complicated. I'd say it is a case by case issue. Advice to people is have your adventures before getting married, after you get married, focus on raising a family.
Cheers!