That is a body double.
Heres an interview where Mila talks about using a body double for her flashing scene in Forgetting Sarah Marshall and how she won't do nudity.
If they don't, then they don't, but I don't particularly want to ever do nudity. I love my parents and my grandparents and there's no need for me to take years off their lives. It's not to say that I'll never do nudity, but at this point in my life it's not something that I want to do.
So there is hope.
Winona Ryder used to say that she'd never do nudity...
"Coffee is for closers only!"
Alec Baldwin, Glengarry Glen Ross