Not sure what to call this

So, there’s a type of sex story that really appeals to me but I’m not sure what to search for. I understand it’s horrible and should never be done irl, but stories are just fantasy. I’m not interested in debating the morals and ethics of it.

Basically, I like the idea of a girl and a guy going at it, but partway thru he switches out with someone else without her knowing it.

What is this called?

Source: reddit.com/r/eroticliterature/comments/4uaon5/not_sure_what_to_call_this