"Filii" should be understood as children of both sexes, not only sons. There are some Old Testament parallels describing a particularly bad influence of foreign wives on Jewish monarchs (see e.g. 1 Kings 11,8), but obviously in Christian times the main danger for the faith was seen in giving away daughters to non-Christian families. The practice was explicitly prohibited for all Christians (not only clergymen) by Canons 15, 16 and 17 of the Council of Elvira (ca AD 306).
When the canon was read at the Council of Carthage in 525 AD, the sex of the object of the prohibition was changed: "Vt gentilibus et haereticis filiae episcoporum et clericorum non nubant" (ed. Munier, 264), that is "The daughters of bishops or any other clerics should not marry pagans or heretics".