From Rene Rodriguez's review of Savages in the Miami Times:

Those early scenes also introduce one of Savages’ most unlikely conceits: The lovely, bird-brained Ophelia (Blake Lively), who prefers to go by “O” and lives in carnal communion with the two guys, letting them take turns with her in the sack, and sometimes just sharing her bed with both of them at the same time. Even for an Oliver Stone movie, the scenario is an awful lot to swallow: What kind of woman would ever agree to this, and what two heterosexual men, no matter how close, would be comfortable with this arrangement? But you go with it, at least initially, because the entire plot hinges on their harmonious three-way union, a bromance with a woman thrown in for sexual release.

I had a harmonious three-way union in Miami once. And while poly relationships—particularly those involving two men and a woman—are by no means standard, Rene, they're not exactly unheard of.

As for what kind of woman would want to sleep with Aaron Johnson and Taylor Kitsch—gee, I don't know. Maybe the awake kind? (Fully conscious lesbians excluded, of course.)