Kanojo to Kareshi no Akarui Mirai

Overview

I'm curious about my girlfriend's past, and I'm pretty sure I'm not alone! Ichirou, a dull guy with nothing interesting about him, has a miraculously cute and beautiful girlfriend. Her name is Yukika, and she is out of this world and wildly in love with him. At the peak of his happiness, one day he meets a man who tells him that Yukika used to be a slut. Ichiro understands in his head that his current life with his girlfriend is important, but he is caught up in a spiral of dark fantasies about his girlfriend's past. Using a VR time machine that faithfully recreates the world of the past, Ichirou makes a forbidden move...!

Source: AniList