According to "the Heir Decree" in the Ritsuryo codes, an Imperial Princess should marry an Emperor or a member of Imperial family who was within the fourth generations of an Emperor, and in ancient times there was no marriage between an Imperial Princess and non-imperial family.
内親王の結婚相手は律令の「継嗣令」では天皇もしくは四世以上の皇族に限るとされ、古代には非皇族との結婚はなかった。