Ironman North America events have change tents where you can strip naked and even do a poll dance to reward the volunteers for their efforts. But please don't. Change you can, but don't assume the volunteers will think your pasty white butt is the finest on this side of the Mississippi.
I proudly showed my skinny white butt in both Madison and Coeur d'Alene, and aside from one other athlete granting me an obvious wink, nobody else noticed because, well, it was normal.
Change as you may.
As for other events, those outside NA, as far as I've read in race reports, the change area in Europe is mixed, so you'll get to see a few fine back sides of the opposite sex, but I'm not sure how it's handled in South America or at some of the other events. I'm guessing it all depends on the area and how uptight they are about their sexuality.
