Okay but consider this spin on the Lucina situation with M!Chrobin: Robin has shape shifting powers as a result of being a Fell Dragon, but because of his amnesia he thinks this is completely normal and that everyone is capable of doing it. Meanwhile Chrom is just ignorant enough of magic that he assumes it's a spell Robin developed specifically to help them clear the hurdle of getting an heir.

Like they where talking about having children and the need for an heir to the Halidom, when Chrom brought up surrogacy as an option (clearly not thrilled at the idea) and Robin just shrugged went 'well wouldn't it be easier if one of us just carried the child? I can do it if you're worried about it interfering with your royal duties.' And Chrom was floored assuming that Robin must be some brilliant master mage to already have thought this far ahead and come up with a spell to answer the issue, and he's all like 'don't feel pressurized at all I wouldn't mind!' and Robin who assumes that everyone can just alter their physical sex easier then getting a haircut, just shrugs and goes 'it's no trouble, and it makes more sense for it to be me- I can still advise you while pregnant, you might have a harder time leading armies if you are'.

Everyone else assumes along similar lines to Chrom, but no one is actually comfortable in broaching the subject directly, so this misunderstanding continues for years, until Morgan casually rolls up to the mess tent one morning as a different gender then they where the night before and EVERYONE freaks out about it except for Robin and Lucina who have no idea what everyone is making a fuss about.

(Lucina knows that she and Morgan's ability to shapeshift slightly is because of Robin but thought that everyone knew, Robin still just assumes everyone can do it. Chrom is having a meltdown).