Reeves' script review has some significant spoilers, and not just the Internet crybaby "Oh no, you revealed a character's name!" kind, so proceed with caution over there. But the movie's premise is that Portman plays a sweet, innocent ballerina taking the lead in Swan Lake, and Kunis plays another ballerina who looks a lot like Portman but is much more reckless and "dark." How or why they end up in bed together, I won't say, and it's probably irrelevant compared to the more important issue, i.e., OMG PORTMAN KUNIS SEX!!!1! Reeves says it's "ecstasy-induced hungry aggressive angry sex"; whether he means "ecstasy" in the sense of being ecstatic or "Ecstasy" in the sense of the drug you take at raves, I don't know. I would guess the latter.
Of course, this is only a review of the script, which has already been rewritten by Mark Heyman from John McLaughlin's original. Aronofsky might shoot it in a way that's far less, um, appealing than it sounds, or he could cut it altogether. Time -- and the Internet -- will tell. In the meantime, gentlemen, start your engines!