Agree. A good mom doesn't tear apart her family.
However, sometimes women that aren't and haven't getting all their needs met for awhile (because kids can't and shouldn't be meeting mom's needs) will turn outside the marriage relationship and pick a married man, the reasoning being the relationship won't materialize into anything that threatens the marriage (in their mind) and they can get their needs met and stay married and no one gets hurt. No one was supposed to find out. It's stupid but that's what I think was happens sometimes. Maybe I'm wrong too, but when I've seen this happen usually the wife doesn't want to leave, she just...well...was weak. Maybe weak character. Maybe family stuff. I don't really know all that.
When they don't want to try or refuse to give up the affair/man that's an entirely different deal.