Married but separated dating
During the proceedings, the fact that a dating spouse is already separated will be noted, but that does not necessarily mean the circumstances of the new relationship will not be considered.
For example, the judge might disapprove of the dating spouse's behavior and develop a bias against them.
That means that where fault is recognized in a divorce case, dating while your divorce is pending could be considered proof of adultery.
Even if you and your spouse are separated, dating before your marriage is dissolved can be used to help prove marital misconduct during your marriage.
It can look like you have questionable morals, even if no misconduct occurred during the marriage.
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.
If the new partner makes the children feel uncomfortable, whether it has to do with the partner personally or it's related to the children feeling that the partner may be at fault or contributing to the separation and divorce, this is likely to have an effect on custody decisions made by the judge.