Ofc a 16 yr old girl wanting to marry a 40 yr old man is normal in this fictional setting.
Stop comparing this fictional world's common sense and morals to present, real world ones.
If sex or gender is your problem, it's not supposed to be since the character's sexes are very easily interchangeable.
I.e. A 16 yr old boy wanting to marry a 40 yr old lady
These manga target a specific audience in Japan, it's not catered towards the international audience in any way whatsoever.
You can bitch but do know that it's your problem, not the author's.