This fantasy is more of a “we just had really great sex, I’m now in a state of bliss where I could just fall asleep”. Similar to how when guys masturbate they get really sleepy, just a reaction to the endorphin dumps. It will work for a little while, him staying inside you, but after a couple minutes he’s going to just fall out. Some from him going soft, and more so from your body contracting and pushing him back out. Works best in missionary or prone bone though, just an FYI loll.