Wise...? We can see, what happened. Something went very wrong in this family, and their future seems to be not better.
Yes, wise enough not to create a rift within the family and to accept adult children´s decisions with whom they want to spend their lives with! Where would in your opinion be the benefit if they would act like 18th, 19th centuries parents saying "if you don´t give up this person I will never see you again, your not my son/ daughter anymore....!" etc.?! The King and Queen are not that theatrically dramatic, it´s simply not in their DNA! A family feud is never a solution an act of wisdom. Of course, everyone is entitled to have an opinion. But it is a huge difference if you say "oh, are sure you do the right thing? Have you considered this and that? did you think of problem x or y ....?" and so on, or if you act like an autocrat, estranging your children from you!
ML could argue she has given up virtually everything, her income as a member of the RF, her title, and even then her new partner would be rejected. In that case I would feel treated very unfair, to put it mildly, myself, too!
No, I´m happy there is a different atmosphere within the RF, despite all the troubles life may bring, than in the belgian or romanian royal families where some people don´t even seem to talk anymore....
"We can see, what happened" No, we can´t, because Mr. Behn´s suicide was not a result of the King and Queen giving permission of her daughter to marry him back then 17 years ago, but the result of his mental illness!
Having had a son-in-law suffering from mental illness could happen to everyone, not because things "go wrong" in a family and you know that!
The NRF was always been a very warm and caring-for-each-other one - but still this fate can hit you. I have heard from families where everything seemed to be just fine, lots of love and compassion, but still a son at some point later in live, meanwhile a family father and grandfather (not seperated nor divorced), daughter happily married, killed himself after years and years of struggling against the desease. There are 100s of examples such as this prooving your argument as completely invalid.