I'm unconvinced of this one. If someone is capable of coherently consenting then there's no ethical issue. People do have a tendency to overplay the "I was drunk" thing though if they think they're going to be negatively judged by those around them. The whole "taking advantage" thing being placed on a guy and whatnot.

That said, I'm stressing the coherent consent part. If they're absolutely fucking wasted it's not much different to using a date rape drug.