But at the same time you never really know a person's story. Maybe something happened to their cover-up at the beach, but they still needed to stop at a pharmacy to pick something up on the way home and there isn't enough time to go home, change clothes and go back to the store. It's a bathing suit, I can think of things that are worse.
When I was in high school I did a foreign exchange program. On the way home my jeans snagged on something on the plane and ripped a line along the bottom of the back pocket, exposing my underwear. All my clothes were checked and I had no US money or credit card or sweatshirt to tie around my waist. I sucked it up and walked onto the plane, off the plane, through the airport to my connecting flight, onto that plane, and through customs and baggage check with my butt cheek exposed. I wanted to go home and I wasn't going to let being embarrassed by my pants stop me.
I go agree with PP about people with their exposed butt cheeks in the parks though. I often see them sitting down on hot surfaces on hot days and think how gross that is. But at the same time, I always wear pants in the park and I wash my pants, so my exposure is very minimal. The only time I'd probably say something would be if that person was actually touching their butt and then touching surfaces, which is something I've also seen. Honestly, I'd just keep in mind that every park surface has probably been touched by someone who just coughed in their hand, didn't wash their hands or just scratched themselves and I wash my hands and don't touch my face or food accordingly.