I just don't see why he has to be hurt over it
Becuase it happened. Him not knowing does not magic away the fact that the foundation of their realtionship has been irrevovkably cracked. The person he is with, is not the person he thinks he is with. He has the right to choose.
And as I stated above, there is no simple, rightous, easy solution that can be offered to her that will tidy things up neatly. Becuase doing the right thing by her husband may mean the worst outcome for the kids.
She has to choose who she is going to fail next. That is an unavoidable outcome of her choice to go in for a cheap knee trembler or two.
When you do something like this your next move puts people you love on differnet sides of the field and you have to choose one over the other. It isn't a case of "protecting" her husband, she failed to do that when her knickers hit the ground. All she can choose now is who out of kids and husband she fails again.
Him by refusing to give him the right to make his own choices with all available information.
Or the kids by risking the collapse of her marriage and possibly years of turmoil and distress for them.
There is no way to gloss over that and pretty it up as an act of kindness on her part. Kindness would have been to chose not to fuck a stranger. Too late to think of herself as self sacrificing now, it's her husband's autonomy and dignity or the kids stability she has to sacrifice.
She shouldn't find it too hard to fail either husband or children again. She managed to forget that they mattered long enough to get dizzy about fucking this stranger again. Her family appear to be viewed by her more as background decoration for her midlife drama than actual human beings anyway.