I feel like so many people share the sentiment that Denji was "lol lobotomized" in part 2 without ever giving thought to the fact that... I mean... Trauma can fuck you up, yeah.
In part 1, Denji learns that not only is he allowed to have affection, but also that that affection doesn't necessarily have to be sex. It's a very important, prevalent theme of the manga.
But by part 2, he has lost nearly everything - Nayuta is all the family he has left, and even that was basically just foisted onto him. He has a sense of purpose and family, but still not in the way he actually wants it. And he walks around shellshocked and zoned out because - in case ya missed it! - he literally had to witness and/or cause the deaths of anyone he's ever cared about.
And so Denji becomes fixated again on the idea of sex - this time not because it's the only form of love he can conceive of, but because it's his "If Only". "If only I had more money, I'd be happy", "If only I were famous, I'd be happy", "If only I lived in a different place, I'd be happy" - people LOVE their "If Only"-s, their ideas that one single little factor is all that's keeping them from finally being happy (and of course, if you ever happen to achieve your "If Only", you're more than likely to quickly find a new factor to blame all your woes on). For Denji, sex is the one thing he wanted from the start (which he still doesn't consciously realise the reason for), it's the one thing he never got, and even though he's experienced family and financial stability and purpose and Not Being Homeless, he still isn't happy - so, yeah, "If Only" he had sex, maybe it'd finally fix him.
I feel the need to point out that Denji not being particularly smart is a key factor here. He can't put together that that's not how life works.
On top of that, he also clings more than ever to the concept of Chainsaw Man, and he does so for two reasons - one, because it lets him escape. Because he doesn't want to be what he is, and what he is is a traumatised, lonely, sensitive, isolated, terrified, miserable boy, and who would want to be that when they could be GODDAMN CHAINSAW MAN?!
Chainsaw Man stops being a tool he can use to chase his dreams and becomes a replacement for Denji. Even when Chainsaw Man comes directly between Denji and his dream of a simple life, he chooses Chainsaw Man - because a simple life has already been taken from him, because he doesn't trust it will stay, and because he profoundly hates being himself. Chainsaw Man becomes an escape, an obsession, an unhealthy coping mechanism, Denji wants to disappear into Chainsaw Man because Chainsaw Man is Cool and Epic and People Love Him, and Denji is sad, pathetic and alone.
Chainsaw Man is strong. Denji is weak.
It's just the best thing to use to escape himself.
And that second reason? Chainsaw Man is also Pochita. Chainsaw Man is the physical manifestation of the first friend Denji ever had, of the first being that ever loved and held him. Of course he can't let go of it, even though being Chainsaw Man doesn't let him speak to or be with Pochita, his grief and his desire to cling to someone that's already gone only strengthens with each new person he loses. Chainsaw Man also represents his refusal to move on and actually live in the present.
And, yeah, the manga doesn't do much to "teach" Denji to do better. Maybe it could have, were Fujimoto a different kind of guy. Personally, I always knew the ending would feel the way it did.
But Denji isn't written like a shonen protagonist. He's written like a person. And sometimes when a person is obsessed with an unhealthy coping mechanism, the only thing to do is to just take it away.
