I assume that the girl is inconsequential, and he uses her a symbol of his "normal life" that he lost with parallel world drama. Getting with her lets him feel like he isn't trapped in a magical NTR hellscape which seems to have monsters in it. It seems like a fairly normal mental trap for anyone dealing with parrallel worlds and time travel- the strong urge to 'make them how they were supposed to be'. That makes people try to force their own alien memories on a world that was going along doing its own thing.Man, there's other girls out there, no use breaking your back over a girl who don't even remember you.
After that gets twisted around for a while, they stop trying to make things "how they were supposed to be", and start making those "how I would personally want it". Which is honestly the same thing, but then you lose the guard rails and just go whole hog into the narcassism and hedonism that closely mirrors our browser histories (since the endless choice of the internet provides a similar style of experience; let's be honest- you are in an NTR and pregnancy thread. You know what I mean)