Isn't FumiDen a perfect example of Denji's ineptitude? As we all know, Reze was in a different world than Denji, but the perfect girl would come to him, and it was Power. So after Denji switched his promise to find Power, with 10 girlfriends, that ship sank, but a miracle arrived in his lap in the form of Fumiko Mifune. A cowardly, fun-loving prankster, who wants to be on the winning side like Power's mentality, but with 2 key differences. Unlike Power who sacrificed her life for Denji and loved him, Fumiko refused to sacrifice herself for him and doesn't love him, and this was all the result of Denji's choices.
On the Fumiko route, Denji made the blunder of sucking the tentacle when they first met, instead of rejecting her and making Fumiko want him more. That wasn't the ultimate bad end because that came in the form of Denji abandoning his family for whores, something that is a no no with Fumiko, as she still has resentment over him not saving her family. Denji had the perfect girl for him arrive a second time, and he ruined it with his own foolishness.