Until very recently, the discussion about what really constitutes the impediment of sexual impotence was wide open. Among those who wrote on the subject was Paolo Zacchia (1584 - 1659), an often-quoted pioneer of forensic medicine in Italy. Zacchia's understanding of the canonical copula different substantially from that which was then current among other authors and canonists. While insisting on the necessity of penetration, Zacchia retained that the ejaculation of the verum semen, with its connotation of being elaborated in the testicles, was not indeed necessary. It is true that for many years this opinion was not admitted. In the end if the praxis and the 1977 Decree of the Congregation for the Doctrine of the Faith proved him right, Zacchia had in turn provided incontestable proof that, contrary to all authoritative allegations, Sixtus' Cum frequenter did not really resolve the question in the sense it was claimed to do and that all along there was indeed much reason to doubt the wisdom of the opinion that was actually followed. This study presents the thought and doctrine of Paolo Zacchia on impotentia coeundi, as well as its forensic implications for the validity of marriage.