Here is an example from the Court. Just a few years back Lawrence v. Texas said that prohibiting private homosexual conduct was unconstitutional. The reasoning was basically that privacy in sexual relations is a natural right. However, those on the court that espouse original intent argued that laws prohibiting various sexual activities, especially sodomy, were in existence in some places when the constitution was enacted, and no one at the time thought they were unconstitutional. Therefore the original intent of the framers was not to protect homosexual relations.
By that logic, slavery could be considered natural law based on the fact that it was legal in some places at the time.