I'm pretty sure this is a oneshot. As far as I remember, the main character is hospitalized. He has quite some fantasy about having sex with the nurse. One night, suddenly a dimension door open, and pretty girls seduce him to going in, with lots of fantasy. I remember exactly the door has vagina shape (and the main character said so (vagina shape)). As he's going in, an old janitor (male) stopped him, saying it will be dangerous as he can never get out, those girls are succubus (kinda like that) and go inside instead of the main. Next is the janitor playing with those succubus, as the door closed. The main char stay alive...

I remember the story is quite interesting, as it's a hentai. Maybe someone has read it?