Erm, no, that might be true in some areas, but it is notably not true in the UK or France, which is where the Arthurian legend was written. So, there is no reason to assume it would apply to King Arthur.
United Kingdom did not exist when medieval reiterations were written.
Neither Englishmen, Frenchmen nor Germans who wrote them at the time approved of children from morganatic marriages being heirs
Englishmen started approving of them in 17th century
Frenchmen never.
Germans never did too.
Erm, again, what? Henry VIII, who lived in the 16th Century, married morganatically
four times, and with the intent of providing an heir. And his son did too. Your statement simply does not hold water, and you have no evidence for it whatsoever. And, there was never any indication prior to that that children from morganatic marriages were considered not to be heirs. It is simply not something that was ever held to in this country, or in France for that matter.
If you think it was, then provide a clear-cut example where it was forbidden due solely to the nature of the marriage.
