That’s some wild-assed hypothetical you’ve got going there. I’ve been part of the drinking/ drug culture, and I’ve known lots of people that were part of the drinking/drug culture and I’ve never heard of the dynamic you described. In fact, getting inebriated when your partner deliberately stays stone-cold sober would be weird, unless there’s some sort of designated driver thing (and most hard drinking / drug culture people aren’t even that responsible).
And having sex with a comatose person isn’t fun for anyone but a rapist.
I may sound like an old lady, but don’t kids these days know how to fuck anymore? It’s all about consent, your partner should be continually consenting with their hands, with their mouth, they should be whispering consent in your ear, if not screaming it. If your partner isn’t actively responding to the experience you should stop and find out why, not because you’re worried about your freaking legal liability, but because you want them to enjoy it. It’s not enough just to get consent before you start, because moods can shift fast, especially late at night, especially when alcohol and/or drugs are involved. Just learn to be a considerate lover and you won’t have any consent issues.
Maybe somewhere out there, couples exist that play some game where one of them gets passed out drunk while their partner fucks them. I did read an anecdote once where a women realized she had sex with her husband after taking an Ambien, and although she didn’t remember it, it was the best sex of his life. And for some reason, probably having to do with incompatible sex drives, they made a thing of doing that.
I would say if you are in a relationship like this, you better trust your partner enough to know they would never press charges against you, cause you’re taking a risk.