In Pennsylvania, when a child is born during an intact marriage, there is a presumption that the husband is the father of the child unless there is evidence that he didn't have access or was incapable of procreation. Where paternity is presumed, an outside party who is the biological father of the child does not have standing to claim custody rights of that child.
Almost everyone has seen at least one episode of the Maury Povich Show or Jerry Springer during which the climactic moment is the announcement of whether or not the hero (or sometimes villain) of the episode is or is NOT the father. This scene is usually followed by the gentleman in question celebrating a result he expected accompanied by the plaintive sobs of the mother who was SURE that he was the father and promised he was the ONLY ONE.
In Pennsylvania, there is something known as a presumption of paternity when referring to a child that is born to a married couple. Under this presumption, the law assumes that a child born or conceived during an intact marriage is a child of that marriage. This means that husband and wife are presumed to be mom and dad. This presumption is very strong and extremely difficult to overcome.
Paternity is often a very important issue in family law cases. Most people have watched episodes of "Jerry Springer" or "Maury" where whole shows are based on revealing paternity test results to a couple, which either turns out to be good news, or a not-so-great revelation that causes fighting on the set (cue the Bodyguards!) However, in the family court realm, paternity is an issue that is taken much more seriously than may be portrayed on television.