JiffyPopTart
Bree-Yark
I agree no sane person would be punching a gelatinous cube or green slime, but martial arts RAW isn't just punching. It can also be kicking, knees, elbows, etc. You could also stretch a bit and say you are kicking a rock at their head or using the environment in some way to hurt the monster. The magical damage issue goes away at 6th level when martial arts damage becomes magical itself.I think there is a thematic problem with being forced to use unarmed strikes, when Kensei is more about using weapons.
As a matter of fact, I have the same thematic problem with the Monk in general, because I do not like the idea that characters are fine with touching monsters with bare hands even when hitting them.
OTOH I wouldn't want to touch the RAW, so what I would do is use all the statistics of the unarmed strike (lower damage, no magic effects, etc.) but narrate it as a weapon attack.
But yes, I refluffed my martial arts attacks as narratively hitting the monsters with the hilt or flat of the sword instead of the slicing edge rather than a punch along with two sword swings as that makes way more thematic sense for a weapon focused martial artist.
I do think the requirement to make a MA attack to get the bonus was odd for the weapon focused subclass.