How have I missed this question for so long?<br />
No, it is not hard. The very first time you attend a naturist event it is possible, but unlikely, that you will get an erection. The best way to deal with it is to go for a swim (especially if the water is cool) or just lie on your tummy until it goes away - it will.<br />
Erections are triggered by the brain and the brain is not stupid - if sex is not on the cards, the brain doesn't prepare the body for it. Nudist events are not about sex, so you should have no problem. If you do, you have gone there for the wrong reasons.