Offense is in the mind of the beholder, true. Some people will tell you nothing. Some people will spell out the difference- in great detail. There is little difference in the two terms as espoused by the American Association for Nude Recreation and The Naturist Society, the US's two largest CO organizations. Americans have been arguing the point since at least the 1930s, while Europeans seem to have little problem using the terms interchangeably.
Most males get erections.
They also occur naturally as the result of external stimuli meeting particular notions of sexuality. Since each male has different criteria for being aroused, it is difficult to assert that this erection was caused by that stimulus. This problem occurs far less frequently you might think. Nude recreation is just not that much of a sexual situation for most participants. As in any social setting, the Golden Rule applies.