And in the UK the rules have always been loosen specially after WW1.
Bertie married equally the daughter of the impoverished and meaningless future king of Denmark.
His elder son, the infamous Duke of Clarence, after triying in vain to marry equally the daughter of the exiled french pretendant, had to conform to a marriage with a morganaut that no true royal in Germany would accept.
Having died before the wedding, his brother, G V stepped on his shoes ... and took his bride too.
Their son Edward VIII, better leave aside, after rejecting all the equal marriages that were presented to him.
Followed G VI who choosed the plain younger daughter of a mere scottish peer.
With a little help of Dickie Mountbatten, Elizabeth II married (genealogically) above her, if one ignores the Battenberg branch.
Charles married Diana, the daughter of an Earl and William married a plain commoner.
Relative to other heads of houses, just how out-of-step with the times was King Umberto in expecting his heir to marry equally? Taking a quick and only superficial survey of Europe, one sees that Bavaria resisted relaxing its usual standards for the heir who had married a countess in 1930. His marriage and children had to wait until 1949 for full acceptance. Belgium relaxed its expectations in 1959 when for the first time an heir was approved to marry someone who was not of a sovereign house. The Netherlands relaxed in 1966 and Denmark in 1967. Umberto's contemporary King Olav struggled a long time before relenting in 1968 to his heir’s marriage to a commoner. Prussia’s heir was less fortunate and had to renounce in 1967, as did a younger brother the year before. The Swedish heir Bertil was only married to his commoner companion in 1976 after his nephew the King married a commoner. In 1975, Prussia somewhat relaxed its usual expectations of the royal heir by accepting a countess, albeit one from a mediatized house. Luxembourg allowed for the heir to marry a commoner in 1980. Hanover followed in 1981. So that is where things stood when “old school” King Umberto II died in 1983.
Post-Umberto, Oldenburg relaxed its usual standards for the heir in 1987, when he married a non-Gotha countess. Otto of Austria held out hope for his heir marrying equally until he relented in 1993 and accepted a rich baron’s daughter after resisting a match with a morganatic Habsburg. Greece relaxed in 1995, the same year the head of Portugal’s RF married a member of the lower nobility. The Bulgarian heir was permitted to marry a commoner in 1996, his father having previously “lowered the bar” by marrying an untitled Spanish aristocrat in 1962.
We are now in the 21st century… Baden relaxed in 2001, as did the Two Sicilies senior line (only after a prolonged resistance). Hesse relaxed in 2003. Spain in 2004, the French House of Orléans in 2009. Parma in 2010. Holstein in 2017. Yugoslavia (for the unmarried heir’s brother) in 2017. And then there’s Liechtenstein, Brazil, Prussia, Saxony (lots of issues there!) and Württemberg, where heirs marrying within the Gotha has generally been the ongoing practice if not the rule. Lastly, if one recognizes Maria Vladimirovna for Russia, she has recently sent what is to my mind at best a mixed signal regarding the status of her future commoner daughter-in-law, so Russia may also figure in this last category.