I think there's a bug now? I know this and it worked properly before. Version Azure 1.4.2. Every time I landed my child, allow marriage checkbox is insta checked and I cannot remove it. After a few years they aren't marrying on their own but I still can't guarantee if it's only visual bug. So I needed to get someone for him to marry just in case.
Last time I played was in Version 1.3 and I didn't have any issue with this. I also remember if they had been landed for 10 years and you have to allow marriage, there was a tooltip when I hover on it saying "It has been 10 years, character_name can marry on their own" or something along those lines. But here I can't uncheck allow marriage right after I landed him. It's checked by default after I landed him and clicking it does nothing. Also there's no tooltip saying it has been 10 years.
EDIT: YEP I'm pretty sure it's at least a visual bug. I disabled all my mods and landed all my sons for science and same thing happened. Can't uncheck allow marriage.
Important bit here the child has to be
unlanded. I tried a new start with West Francia, their children is already landed and the allow marriage box work properly and I can uncheck it. But if you start with unlanded child and land him then the bug happened. Try by playing Duchy of Bavaria in 867. Land your unlanded child and the bug will happen.