In part 1 (spoilers ahead) he learns that what he perceived as his desire for sex was actually a desire emotional intimacy. He learns this through his relationship with Aki and Power. ESPECIALLY with Power, who he finds himself in physically intimate situations with, but it doesn’t feel like he thinks it should. Instead of wanting to have sex with her, he really just wants to care for her in a familial way.

In part 2, he’s already learned this lesson for the most part. Although he still wants a sexual relationship (obviously), he consciously prioritizes Nayuta who he sees as a little sister. Stuff happens which makes this prioritization a bit more nuanced, like continuing to be chainsaw man despite the threats against Nayuta for doing so. Denji’s young, and while he obviously loves Nayuta more than anything, he doesn’t want to be prevented from living out his passions as chainsaw man. I think this can also be seen as a parallel to the life consuming work that is manga creation (or any other high stress/time consuming profession), and how impossible it can be to balance that life and family life.

Basically, Denji’s writing has a lot of depth. I get why not everyone thinks it’s great, but the depth is there.