Elisabeth is the only one who will have to marry someone titled (unless the law changes) but there's plenty of "lesser" nobles in Europe..... apart from that I can see most of them following in their parents footsteps and marrying a "commoner"
I'm pretty sure that by the time Elisabeth is at an age where she wants to get married, a law like that would be changed so she could marry who she wants, provided they are suitable (and not a Sssssslitzerella sort). I can't see a country of people in 2032 supporting telling her she can't marry who she loves and she has to go and find some noble to marry instead.
Such a law doesn't exist and never did.
Belgian heirs to the throne need the consent of the government and that's it. With one curious exception: if Elisabeth decides to marry Constantijn's son or one of the Van Vollenhoven boys a decree from 1830 needs to be abolished.
3. No Dutch royals on the throne
Have you ever been so mad… you banished someone for all eternity? That’s basically, on a national scale, what a decree of 24 November 1830 did.
The decree debars members of the House of Orange-Nassau (basically: the Dutch royal family) from any position of power or public authority in Belgium, in perpetuity. In theory it not only excludes members of said House from the throne, but also from access to the public sector and from standing for elections.
The decree is considered to have constitutional status, rendering its abolition quite difficult. It goes back to the period immediately following the rule of the Dutch King William I in what is now Belgium, from 1815 until 1830. Suffice it to say that the Belgian revolutionaries found that William’s rule left some room for improvement…
http://www.fansofflanders.../11092014_JogchumVrielink 