This makes me uncomfortable.. and disgusted

Honestly, same.
8 y.o boy nurse his wife when she got sick, spoke to his "wife" about being reliable husband etc.
And the "wife" 16 y.o girl treat the boy as her "husband", not merely as a boy.
What's wrong with the author??

Marriage of Convenience also has big age gap with the bride age under 12 y.o but the bride and groom did not have this kind of chemistry. The groom simply stay away from the kid until she reached adulthood. Their dialoque when they were young is very minimal, they did not talk about husband-wife duty.