Thank you for the information. Who were the prime backers and opponents of equal primogeniture in the 1970s? If equal primogeniture had been enacted in the Constitution of 1978, would Felipe have kept his place, as with Crown Prince Harald and Prince Haakon of Norway and Prince Albert and Prince Philippe of Belgium?
In Norway, Harald's sisters never had succession rights. Haakon indeed kept his place in line ahead of his older sister princess Märtha Louise (as both of them were in their late teens).
In Monaco, male-preference is still alive, that's why Jacques is ahead of his older sister Gabriella. So, prince Albert has never been 'at risk' of loosing his place.
In Belgium, prince Philippe is the eldest child of king Albert and queen Paola, so, there was no way he could loose his place to a younger sister.
It is interesting that in European monarchies, the opposition to repealing male preference or semi-Salic/Salic law seems to be more far-reaching amongst male royalty (others include King Carl Gustaf of Sweden, Prince Albert II of Monaco, Prince Laurent of Belgium, Prince Petros of Greece, and Prince Knud of Denmark) than female royalty (Queen Silvia of Sweden is the only one I know of).
How do we know all of them oppose repealing male preference?
King Carl Gustaf is indeed a known example (I didn't know about queen Silvia - but she is only a consort, so her opinion is not as relevant in the greater scheme of things). As I just posted, he was king because of the rules, so, it's rather logical that he was identifying with his son and wanted him to be king just like he was king instead of his eldest sister queen.
Prince Albert is also the reigning prince because of the current rules.
Prince Laurent just wanted to keep his place in the line of succession and not be moved down (which most likely was an important reason why they wanted Astrid and her children in).
Prince Petros of Greece I don't know, but if you mean crown prince Pavlos; again, his elder sister would be crown princess if the rules had been different.
Prince Knud and his eldest son would have been kings, had the rules remained the same, so again, very logical that he opposed as it was HIS birthright that was taken away from him.
I assume it will be changed if and when Leonor has a daughter, then a son?
They were looking into it when Letizia was pregnant of Sofía. It is assumed that the gender was revealed during pregnancy, so it was clear that a change wasn't needed (as revising the constitution would open up a can of worms ). So yes, I assume that if Leonor's first born is a girl and she is pregnant with a second child they will look into it again.