Taking advantage of someone when their drunk is wrong. Even if they where the one who initiate it, it is still wrong. If they make the same offer when sober, then sure go for it. Otherwise its a no-no, I think it may even be classified as rape.
"A man who gives in to a woman offering herself freely when she's drunk or otherwise incapacitated can go to prison because he's committing rape."
Fixed it.
Few things to say here. First, there's a difference between intoxication and incapacitation that probably needs to be considered before calling it a potential rape because there's alcohol involved. Not all laws in all places are as caught up on the fact that women can also take advantage of men as they probably should be despite ample time for lawmakers to get their shit together (have you ever seen this poster? It's pretty infamous for the double standard on it, take a look
). Probably some issues there to be concerned about since laws aren't always perfect or even likely to ever be completely perfect.
Second, what age are either of these characters? I already forgot if his was mentioned here at all, but he's in a high school uniform from the looks of it and by comparison she may already be a legal adult by our standards. If we're gonna start this conversation we shouldn't ignore the fact that she's potentially a statutory rapist if they go through with it. It's not like one of us could go get blackout drunk, sleep with a minor and not get in trouble for that if it's discovered.
Lastly, this world isn't even our world. We have no freakin' clue whether either of those things are in any laws at all over on random isekai planet #32,687. Let alone in this specific part of that world, and I mean our own world doesn't necessarily have the same laws on both subjects in every location so why should they? Whether we find either of those points morally disagreeable or not, if it's not a crime in the world they're in, neither of them will do the time. At that point you can pretty much find the material offensive if you want but, that's not really going to change anything except perhaps whether to continue reading it or not. That's pretty much the entire weight of any moral issues brought up here, a simple decision to read it or not.