@feha
My cousin said they wanted to marry me after I played (forced to on my half) with them for half a day in a garden. They were around 5 and I was around 10. In fact, they pinned me down before saying that.
Besides, it's not like parents go around explaining what marriage is to their kids randomly - they explain these things when they're asked about them. If for whatever reason they're not asked, they can't exactly help remove misconceptions.
Anyways, your point on her being royalty is decent, though even the King didn't wanted to marry his daughter to the OP MC, so maybe it isn't the same as with others.