Although past movies with lesbian scenes have done well, it is no indication of how future movies will perform with audiences.
So I'm only paraphrasing what Natalie Portman said about her new movie.
I guess it's one way to peak the public interests in a movie about ballet. It was either that or have the plot be about a Russian dancer who defected from the USSR to the USE end up in the USSR again. But that's been done.
I'm not trying to single out Black Swan (which I haven't seen and probably won't. Not when I have yet to see Tron: Legacy and The Chronicles of Narnia: Voyage of the Dawn Treader yet). But I doubt that guys will fork over the nine bucks to see the movie for that reason alone. Not when they are never more than three clicks away from porn for free on the internet*.
The lesbian scene is a gimmick. It was added to create buzz about the movie. Mainly from when I read anything about it, the girl-girl action is always mentioned. Would the movie be talked about if it wasn't?
Related: It's a tell all book so take it for what it's worth but-- if true-- does confirm much of what people suspect about Hugh Hefner. Thanks to Ace for that last link.
*A statistic I made up on the spot but it sounded good.
Updated: Linked by The Other McCain.