Basically they would qualify -- sort of like Queen Anne's husband. It's really tricky, because for the good part of history infant and childhood mortality rates have been extremely high.
When in my earlier post I said "Youngest sons", I really should have said "LAST-born sons". But then, because in many cases a sovereign had only one younger brother, and that said brother died in infancy, perhaps he should have counted, after all. Kings Henry VIII of England, George V of Great Britain, and Alexander Karageorgevich of Yugoslavia all grew up as functional youngest sons.
But by that same logic, a prince could have had an older brother who died in infancy -- perhaps before he himself was born -- in which case he would have been a functional eldest son. I believe this characterized Czar Nicholas II of Russia.
As it was, I couldn't quite make myself clear, and the answers all ended up varying as a result.
Are these acceptable?
Francisco de Asís, Duke of Cádiz (1822 – 1902) the second son (first to survive infancy) of Infante Francisco de Paula of Spain, and of his wife (and niece), Princess Luisa Carlotta of the Two Sicilies.
Francisco de Asís, Duke of Cádiz was the king consort of Queen Isabella II of Spain from their marriage on 10 October 1846 until Isabella was overthrown on 30 September 1868
Henry Stuart, Lord Darnley (1546 – 1567) was an English nobleman who was the second husband of Mary I, Queen of Scots and the father of James VI of Scotland and I of England.
He was the second but eldest surviving son of Matthew Stewart, 4th Earl of Lennox, by his wife Lady Margaret Douglas.
Margrethe I (1353 – 1412) was Queen of Denmark, Norway, and Sweden (which included Finland) from the late 1380s until her death, and the founder of the Kalmar Union that joined the Scandinavian kingdoms together for over a century.
Her husband was King Haakon VI of Norway who was the second son of Magnus Eriksson, king of both Norway and Sweden. His older brother Eric XII was meant to succeed their father on the throne of Sweden, while Haakon was made king of Norway.
Who were the royal princes born as second sons in their families and who married women who became heads of royal houses?
Probably the best-known example would be Prince Albert of Saxe-Coburg and Gotha, who in 1840 married his first cousin, Queen Victoria of Great Britain.
Their kinsman, Prince Gaston of Orléans, married Isabel, the Princess Imperial of Brazil
Prince George of Denmark (1653-1708) was born the third son in his family (his father being King Frederik III); but because one of his older brothers died in infancy, he might count as a second son. In any case, he was married to Queen Anne of Great Britain.
Are there other examples? What about the early pre-Spanish kingdoms or Navarre?