The UK rules perhaps, but *the practice* for three centuries had been that approved marriages of all who were the immediate heir at the time were with persons of princely rank. The first immediate UK heir to break the pattern and marry a non-princess was the current Prince of Wales, as his grandfather George VI was not the heir when he married the late Queen Mother.

So, in the very early 1970's when Umberto II was struggling with his son VE's determination to depart from a long tradition, he was in good company among monarchs and would-have-been monarchs who had not to date given ground on this issue:

Italy
UK
Liechtenstein
Brazil
Württemberg
Saxony
Saxe-Altenburg
Prussia
Sweden
Luxembourg
Romania
Hanover
Oldenburg
Austria-Hungary
Austria-Este / Modena
Greece
Portugal
Reuss
Hohenzollern (one exception, out of political expediency, for Antoinette Murat in 1808)
Russia (GD Vladimir was not an heir when he made what some deemed an unequal marriage)
Bulgaria (King Simeon was not an heir when he married a non-royal)
Montenegro (Prince Michael was not an heir when he married a commoner)
Mecklenburg (heir's 1941 marriage was deemed morganatic in some GHdA editions)
Baden
Two Sicilies (senior line)
Hesse
Spain
France (House of Orléans)
Parma
Holstein
Waldeck
Yugoslavia