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Usually, for one of two reasons:
#1. Sure, even though it’s only a couple of extra minutes to go grab one (if one’s available) and put it on, often we just don’t want to spend the extra time when we could be swimming and goofing off,
#2, Often we’re somewhere where, while it’s private (like at a friend’s house) not everyone happens to have a swimming suit, and they don’t want to miss out on any of the fun by running home to get one (again, kind of like above), and we don’t want to make them feel singled out by telling them they have to go naked while we aren’t (and no one want to go around in wet undies afterward). So it’s a courtesy that if people don’t mind going the buff we all do it together. Of course we aren’t forcing anyone to go naked if they really don’t want to, but again, as has been pointed out, it’s usually where we’re all dudes who have already seen each other naked many times already (in the locker room at school for friends, or in the bedroom with brothers), so there’s really nothing to be embarrassed about when it’s just us.
