A more thorough explanation:

Lord Mansfield's rule is a legal principle that says neither spouse can testify about whether the husband had access to the wife at the time of a child's conception. This rule made it impossible to declare a child born during a marriage as illegitimate.

For example, if a married woman gave birth to a child, but her husband was not the biological father, the husband could not testify in court that he did not have access to his wife during the time of conception. This rule protected the child's legitimacy and prevented the husband from being publicly embarrassed.

However, most states have abandoned this rule, and now DNA testing can determine the biological father of a child, regardless of the parents' marital status.