Though it might be legally relevant, I try to avoid having my clients focus on the bad behavior of their spouses that pre-date the marriage (or the bad behavior of my custody clients’ co-parents that pre-dates the child’s conception). Even if that behavior is relevant, focusing on it highlights the client’s bad judgment, along with an apparent lack of grace. Often pointing out this bad behavior causes more harm to the party pointing it out than the party who behaved badly so long ago.
No matter how disturbing the fact that the other party was a cat strangling, child molester (or a cat molesting, child strangler), one’s own client decided to marry that person or have a child with that person. Even if that behavior continued after the marriage, or after the child was conceived, I try not to focus on it unless it is directly relevant to the issues in the case. Otherwise, my client looks foolish for forming a family with someone he or she thought so poorly of, as well as churlish for highlighting misdeeds that pre-date that family formation.
And when the other party complains about my client’s pre-marital or pre-conception behavior, I have an easy cross-examination retort, “That behavior bothers you so much?: Well you married ‘em [or bore a child with ‘em].”