So this can be consider cuckoldry.
Someone or something else is using the mc body to have sex with the girl that the mc want to have relationship with.
Best example of a similar thing I heard of is 1 super hero comic book about wolverine swap body with spiderman.
And go on to take the first time of spiderman girlfriend using spiderman body.
Spiderman was completely unaware and only realize after the gf mention it.
She was also unaware btw.
But it is still considered cuckold/ntr.
There is also a pornhwa about a guy ntr-ing his past self. Basically his gf time travel into the future and have sex with his future self, then went back in time with no memory of it. That mc live with that doubt needle for decade until the event happen to him to make him realize it. And that mc is the type that only get hard for his gf too, went limp even when a girl try to rape him. He is that loyal, and has to live for decade in doubt about his gf first time. They were childhood sweetheart too.
Similar case to this manga, so yeah, ntr kind of becoming a term to use when you feel betray by someone you consider partner or future partner, it refer to how you feel. Different from it dictionary definition but more accurately describes what happen