Certain prohibited relations or transactions may still give rise to ownership or other consequences under specified legal rules. That does not make the prohibited act permissible; it distinguishes the norm governing conduct from the rules governing legal effects.
The text includes historically situated examples involving family ownership and intercourse during menstruation. Their detailed outcomes belong to specific juristic frameworks and cannot be applied without their conditions.
“Forbidden” establishes the prohibition of conduct; claims about ownership, lineage, or other consequences require their own supporting rules.