When a married woman fornicates (i.e., has sexual intercourse outside her marriage), she emotionally cuts herself off from her husband. That’s how she’s able to act thusly.
In addition, her unfaithfulness reeks of contempt and disrespect for the one who’s her God-appointed authority. Consequently, she not only spits on her husband but on God as well.
Her newfound emotional and sexual attachment indubitably engulfs her to the point where her disposition will manifest itself down the line with further infraction. Very rare are these women, if any, whose married lives go back to normal.
Still, if her husband forgives her and does not wish to divorce her, whether it is because of their small children or some other reason, then that’s between the two of them. There’s NO biblical law that says he MUST divorce her – even though the law openly allows for that possibility.
Now, whether their love and their trust in each other, which create family harmony, will remain unscathed, especially on the husband’s side, is quite doubtful: that’s why such an instance is the only time God, who knows His creation quite well, allows a man to divorce his wife.
The following is an excellent primer that’s really worth a look: Of Fornication, Divorce, and Adultery.