[Is there such thing as consensual sex with someone asleep? Like, someone has a kink for having sex with unconscious people and they ask their partner if they could sleep with them after their partner falls asleep.]
Legally, sleeping people cannot consent. And consent cannot be given in advance for a future situation.
That being said, whatever partners do in their bedrooms is up to them. If they both are comfortable with the situation you described, then I don’t have any issue with it.