If you start dating the person immediately following a separation and suspicions were there, then of course this will cause your spouse and his or her attorney to dig further.
If you actually did have sexual relations with the person, then post acts of sexual misconduct can be used to corroborate allegations of pre separation misconduct. If there is an alimony claim involved and you are the spouse that needs money and you have had a sexual relationship with a 3rd party prior to separation, then your alimony claims is barred if this can be proven in court.
And if the person you are dating is spending the night when your children are present, a judge very well many not like this and it could affect how much time you have with your children if you have to go to court.
Judges like stability and consistency for children and allowing a new person to spend the night and immediately become part of a child’s life so soon after a separation rarely looks good in court.
2) You start dating someone after the separation that you did not have any relationship with pre-separation This wouldn’t necessarily hurt your case if you did not have illicit sexual relations with the person before you separated.