Mary-Kate Olsen refuses to be seen in a bikini, because she doesn't want people making fun of the fact that she looks like a skeleton in an anatomy lab. Said Mary-Kate:
I would love to be able to swim in the ocean in Malibu. But that is asking for a bikini shot. That's inviting something that I don't want to happen. I don't need to be on a "Who's Skinny, Who's Fat, Who's Looking Healthy, Who's Not Eating?" list.
She doesn't mind being put on a "Who's Clueless" list apparently.
(source)
