As for "Age gap" we're very far from a Itou Hachi.
Minami is 18 and Iori is 28. There's a gap, but the plot is about how the couples are "mismatched" and still work somehow. That's what Mikanuji wants to write about. Not about "dangerous" relationships.
Notwithstanding Japanese laws about drinking, they are both adults and a 10 years gap isn't anything to frown about. There a plenty of "legal" couples with such a gap or more and nobody will cry foul.
And as for their sex drive, it goes back and forth. There has been instances where Minami was the one initiating, so she does have a sex drive. Iori is maybe more crude about it, but that's part of her characterization.
Imo, the recurrent abuse is more problematic than their libido.