For example, the judge might disapprove of the dating spouse's behavior and develop a bias against them.
While such a bias is ostensibly unacceptable in the U. legal system, judges are human and biases are natural and even probable in some instances.
In addition, in some states the new relationship may be considered in the division of property or alimony determinations, so the dating spouse may not get as much as they want out of the divorce depending on the new partner's financial circumstances.
This is especially true if the dating spouse begins cohabitating with their new partner during the divorce process.
A divorce and the prospect of a new partner replacing them is often too much for a spouse to cope with, and may cause disagreements and unwillingness to come to a compromise with regard to custody arrangements.
When left up to the court, the judge will make a determination as to which parent should have primary physical and legal custody based on the best interests of the children, and if there is a real or perceived discomfort with the new parter experienced by the children, it is very likely to effect the amount of time each parent and particularly the dating parent is awarded.
The cheating spouse and the third party do not necessarily even have to have a sexual relationship – in some places, a family member who convinces one spouse to leave the other might be liable for alienation of affection (though this is very uncommon).