There is a problem I foresee in the European monarchies in the coming decades, namely that requirements to obtain approval for marriages of persons in line to the throne will no longer fulfill their current primary purpose - preventing an unsuitable person from representing the country as its queen consort or prince consort - now that unmarried partners are increasingly accepted even at the highest official and diplomatic levels of representation in Europe.
I will illustrate my point with an imaginary scenario involving the British royal family because, firstly, the majority of posters on this forum are probably familiar with the Prince Andrew controversy, which made it obvious that persons who have close ties to sex traffickers are regarded as unsuitable to represent Britain and its monarchy, and secondly, a recent British prime minister (Boris Johnson) was permitted to live with his then-girlfriend in his official residence and to have her as accompany him on official engagements before they were engaged or married.
So as an entirely hypothetical example, imagine a scenario in which Prince George of Wales, who is expected to succeed one day as King of Great Britain and Northern Ireland, grows up to date a woman known to be involved in sex trafficking. On the basis of the Andrew precedent, the British public would be angered at the idea of the sex-trafficker girlfriend marrying George and becoming their Queen. But if the couple did marry, that worst-case scenario could be avoided. Simply withholding official consent to the marriage would automatically exclude George from the kingship, and thus exclude his sex-trafficker girlfriend from becoming Queen.
But there is a loophole: If the couple never legally married, the sex-trafficker girlfriend would not be called Princess; however, on the basis of the Boris Johnson precedent, she could live with Prince George in the royal palace and accompany him on all his official engagements, including state dinners, official tours, and so on. Once George became King, his sex-trafficker girlfriend would represent Britain at the highest levels as she accompanied him on state visits, international summits, etc. The result would be that, despite the British public's disapproval of having people involved in sex trafficking carrying out official duties, they would have a sex trafficker acting as the first lady of their country and being treated as queen consort in all but name, and there would be nothing they could do about it.
It doesn't seem right to me that European royal couples could circumvent public opinion in this manner, but that seems to be the current legal situation.