Thomas Aquinas

“ I answer that, An illegitimate child can be legitimized, not so that he be born of a legitimate intercourse, because this intercourse is a thing of the past and can never be legitimized from the moment that it was once illegitimate. ”

“ Ignorance, unless it be affected, excuses unlawful intercourse from sin. Wherefore those who contract together in good faith in the presence of the Church, although there be an impediment, of which however they are ignorant, sin not, nor are their children illegitimate. If, however, they know of the impediment, although the Church upholds their marriage because she knows not of the impediment, they are not excused from sin, nor do their children avoid being illegitimate. ”

“ In like manner it is no punishment to an illegitimate child that he has no right to that which belongs to the legitimate children.
Reply to Objection 2: Illegitimate intercourse is contrary to the law, not as an act of the generative power, but as proceeding from a wicked will. Hence an illegitimate son incurs a loss, not in those things which come to him by his natural origin, but in those things which are dependent on the will for being done or possessed.
Art. 3 - Whether an illegitimate son can be legitimized? Objection 1: It would seem that an illegitimate son cannot be legitimized. ”