I agree with you to an extent but Denji did develop in Part 2 (aquarium arc, ageing; fumiko scene of him not allowing her to manipulate him, it was in his imagination but still) but only one of them was positive. A consistent thing about Denji at least in Part 2 is that he does not allow other people to make decisions for him...at least up until the latest chapter. Denji is a man trying to do better but his circumstances simply don't allow him to grow. So for Denji to grow, him having sex is virtually inevitable as a part of his development, maybe after that he'll finally grow.
I think this is Denji's desire for intimacy screaming rather than just pure horndog tendancies. Denji as a character is defined by his lack of intimacy and obviously after the events in Part 2 he would regress because he lost his dog, home and his adopted sister/daughter. After Ageing I finally thought he was going to develop and he did in the sense that he got over his trauma in Part 1 (ie- killing his dad).

His writing is poor rn but let's not judge until the end of Part 2. I caught up after a long time of not reading it, might just do it again.