Moral/ethical values change with time, age, situations, country, religion, and from man to man. Every actions have its own pros and cons
Consider the possible cases:
Case 1] A son (16 or 18) comes to know about his mother's affairs (age 40 to 45). How will he feel? In most cases - Betrayed, becomes rude, violent behavior, addicted, disobedient and destroys his education and career (what I have seen in my family). Do people still think that this mother can control her son? Sorry to say she can't.
She has to have an open conversation with her son. If she loves her son truly, then she has to follow what he demands.
A] She has to stop her affair if son sticks to his point, (otherwise face the consequences.) Or
B] If he wants to bring his girlfriend home. (Win win situations for both mother and son, you live your life let him live his.) Or
C] Son may demand to participate in your Sex life ( which mom may not enjoy that idea, at least she can continue her affair, can control her son. Still win win situation).
Case 2] Where mom and son are very open minded or become open minded after her affair. Share every idea between them. Enjoy having sex or having sex in front of each other with their partners. And the family is happy, respects each other, no jealousy, no fighting... Achieving the goals of their life, then what is wrong with that?
Now we have to think what is the goal of a family. Now ball is in mother's/ parent's hand. Having sex with children and create a happy family or not having sex with children and create a dysfunctional family.