Another reason for the myth of Owain Tudor
Catherine de Valois’ first husband was Henry V, who was clearly the grandson of John of Gaunt. Edmund Beaufort, later Duke of Somerset, is traditionally regarded as Gaunt’s grandson as well, although his father may have been a legitimate Swynford.
Quite apart from the 1420s legislation banning royal stepmothers such as Catherine from remarrying without her adult son’s consent – and we know that she died before he attained his majority – this created an additional barrier of affinity in that her late husband and new partner were apparently first cousins. Such a marriage, quite apart from being illegal in England, would have required a dispensation. No wonder it was more convenient to portray her servant Owain Tudor as the father of her children.
See here for a further complication.