@sodumblol so i can assume you are a male? while me being one too, i was raised with a lot of sisters and aunts (have 3 smaller sisters) and i can tell you that they do think and are very serious of what they learn, marriage for example, is a serious topic, the reason one doesnt take them seriously, its because they will eventually forget about it, but the mc of this, the fl is an adult inside, she wont forget and its not that uncommon in their culture to be engaged while a child or even before being born. Also important to say that engagement isnt the same as marriage, an engagement is a promise and can be broken. the girls family while being an aristocrat accepted because she made a serious mess, they had to mobilize some serious units, they do not have the freedom to do it every time she makes an scandal, so its better to accept the engagement that can be later broken. the kids family doesnt have anything to lose with it, and its a very profitable opportunity, no reason to decline.
you say that your vision of "realistically" cant have these kids talking about marriage, its because your reality is different, maybe you never met that situation in real life, but please tell me, while is kids being serious about marriage, somethig that completely blows your mind as "irrelistic" but magic and statuses and beastkins etc etc etc not?
i think you are focussing your attention in something that you shouldnt, its not that weird, it happens in real life, it dosent go against the setup presented, whats the problem?
edit:typo