In answer to your question about arousal.
Many young teenagers tend to walk around with a towel, this covers two issues. As you say, some young chaps don't require anything to stimulate them, in particular if they are trying to chat up a girl. Girls too at the same age often prefer to be a little restrictive and may carry a towel or wear bikini bottoms. For the lads this covering up also removes some teenage comparisons, where they may feel vulnerable.
Adults: I have no real answer about erections. I have been a naturist for 40 odd years and the number of times I have seen an adult erection would be less than a handful.
Don't know why, it either just is that way or every man must be hiding very well. Bottom line is that although it may be hard for some to believe, naturism just is not sexy that way.