Their angle is the Decree Nr 5 from 1830 which forbids unions with anyone from the House of Orange-Nassau. When it comes to an engagement, without any doubt that Act will be repealed. It is not 1830 anymore (the first year of the secession of the Southern Netherlands).
https://www.hln.be/royalty/zijn-pri...ands-koningshuis-met-elkaar-trouwen~afbb63a7/
And the newspaper De Morgen also about the Decree Nr 5 of 1830:
https://www.demorgen.be/nieuws/neen...30~bfbb63a7/?referrer=https://www.google.com/
Declaration of the National Congress on the perpetual exclusion of the House Orange-Nassau from all power in Belgium (Decree No 5 of November 24, 1830)
On behalf of the Belgian people,
The National Congress declares that the members of the House Orange-Nassau are, in perpetuity, excluded from any power in Belgium.
Declaration of the National Congress on the independence of the Belgian people and the exclusion of the House Orange-Nassau (24 februari 1831)
The National Congress,
Declares that it -as a constituent body- wishes to continue the Decrees of November 18 and 24, 1830, relating to the independence of the Belgian people and the exclusion, in perpetuity, of members of the House Orange-Nassau from all power in Belgium.
Entrusts the executive power with the execution of this Decree.