Or maybe, and hear me out here... he just doesn't want to fuck a minor.
Even with her... particular condition... she is still a minor, her immaturity and situation poison her notions about intimacy.
That is not being a cuck... that is being a man, because a real man doesn't lust over a girl almost half his age.
And one more thing... using her condition to force him to have sex with her is rape... no, i don't care about "arguments" of how it is different, she may be too young and oblivious but holding that over his head is coercion, and if it is coercing him to have sex that is rape... PERIOD.
I find this story cute sure, i like the characters sure... but if he sleeps with her i will be pissed, Japan seems to eager to normalize stories like this, which have an uncomfortable step in real life.