Do you expect a picture perfect body in a bikini? Or are you OK with some fat on legs/tummy? Do you think a girl looks the best in a bikini, with some sort of shorts on the bottom, some kind of shirt on the top, or completely covered up?
If it's a girl my age, around 25, or younger then I would be pretty surprised to see her in a one piece bathing suit unless she is really overweight. There are a ton of styles which can flatter girls of different shapes without covering the entire midsection. As for belly fat, I think that it's pretty well understood that not everyone is a super model.
I'd say there are a few guys who haven't seen many girls in their underwear and stuff who get a shock when they see girls in swimsuits, because so few people are as skinny with curves-in-all-the-right-places as the supermodels they see on TV and stuff!
Personally, I'm over it, they'll just have to accept it. However, what I do find embarrassing is swimming in a swimming pool with a boy where you have to wear a swimming cap! =P