Catholic Encyclopedia (see
here):
The incidents of the long final campaign, which began indeed even before the time of Pope St. Leo IX and lasted down to the First Council of Lateran in 1123, are too complicated to be detailed here. We may note, however that the attack was conducted along two distinct lines ofaction. In the first place, disabilities of all kinds were enacted and as far as possible enforced against the wives and children of ecclesiastics.
Their offspring were declared to be of servile condition, debarred from sacred orders, and, in particular, incapable of succeeding to their fathers’ benefices. The earliest decree in which
the children were declared to be slaves, the property of the Church, and never to be enfranchised, seems to have been a canon of the Synod of Pavia in 1018. Similar penalties were promulgated later on against
the wives and concubines (see the Synod of Melfi, 1189, can. xii), who by the very fact of their unlawful connection with a subdeacon or clerk of higher rank
became liable to be seized as slaves by the over-lord. Hefele (Concilienge-schichte, V, 195) sees in this first trace of the principle that the marriages of the clerics are ipso facto invalid.