Guys im a curious virgin, is sleepin while its inside possible irl or its just a delusion

Sleeping while cockwarming is possible but can get very uncomfortable, it's similar to sleeping with a butt plug in, it isn't recommended.

Realistically, the guy’s dick would probably go soft and would not stay in the whole night

Additionally, it is recommended after a "round" to pee & clean the area so as to prevent infection to either partner. Also, in very rare occasions, may lead to cramping and getting locked.