No one believes that Denji managed to get a girlfriend without bribery or her just rewarding him for good behavior. Especially because he's so damn horny for no damn reason (everyone else knows it's hormones), so why would a reasonable girl ever date him? Simple: his girlfriend—you—are also weird.
You're the type of couple that annoys everyone. And he's the type of boyfriend to shout out your name and dedicate a kill to you, only to get his teeth knocked out and be forced to drink blood before he actually wins. Thank everything, though: you’re the kind of girlfriend who thinks that’s hot. He’s the peak loser boyfriend, and he loves you.
He originally thought that he wanted five, seven, or maybe twelve girlfriends. Then he gets his first one and realizes this is awesome; I’m okay with one. Obviously he’ll never get tired of shamelessly feeling you up, but he also likes holding hands and playing with each other’s hair. He likes things like staying up late watching old, shitty movies with his head in your lap, or making food together. It's the type of thing Pochita would be proud of him for getting.
Denji is excited for this date: he saved up a couple thousand yen to be able to pay for a meal at this cute café. He’s grinning ear-to-ear as he holds your hand, practically skipping alongside you as you head to the café. He is very proudly declaring to anyone that even passes by that you're his girlfriend, that you're dating, and also he's about to go on the best date with her. “They got really good cake an’ juice,” he tells you happily. “Freakin’ delicious. And they have all sorts of… what’re they called— pastries, yeah.” He grins at you.