Aside from sneaking away from everyone else when on campouts with other Scouts and stripping down to be naked alone in the wilderness (I had just always wanted to, and it turned out that I loved the feeling), my first experience with nudism was at a retreat for people interested in polyamory.
The retreat was held at a clothes-optional resort, and a lot (but not all) of the attendees and some of the presenters were naked. Some of the nudists were beautiful by conventional standards, but it was also a new experience to be around people who wouldn't be considered "fit," but who were obviously very comfortable with their bodies: including a very large woman with enormous, sweetly swelling breasts over an even larger tummy, and a beautiful untrimmed bush of salt-and-pepper hair. I didn't stare, but I looked at her for a few moments and she smiled...she liked being appreciated.
Late Saturday evening, I was talking with a woman in her mid-40's who had participated in one of the same sessions I had, and after a good, comfortable exchange about love and sexuality, she asked me if I'd like to swim with her. She stripped completely (she had lovely breasts, smooth full hips, and beautiful, thick curls of auburn hair above her sex) before getting in the large lit pool, and I followed suit (or more properly, I dropped my suit.)
What evolved was a kind of lightly erotic dance in the water...she would hold me briefly, spin in my arms and splash lightly, dance holding hands (she was leading), bring us close together, touch my hips or chest lightly, then spin away again, and repeat. We weren't going to have sex, that much became clear...but it was very arousing. I became very erect, but the water reached to my lower chest, so it wasn't obvious except to her.
When it came time for us to part, she left the pool without a trace of shyness...but I had never been naked in public before...and I had a rock-hard erection!
I swam over to the ladder and, taking a deep breath, pulled myself up and out of the water. It was a new experience and I was a little embarrassed...there were plenty of people around who were half-dressed and a few dancing by the pool who were fully naked, but I seemed to be the only guy standing at attention.
I asked myself, "Why the Hell not?" and decided to stay naked for a while.
I should mention that this was an adults-only event, and there were no rules there against having erections. Sex for one, or consensual sex for two or more, were fine, though not in public areas. (The rules were about respecting other people and never forcing one's attention on them. No problem.)
The Loving More retreat was a place for exploring new feelings and experiences, so instead of following my first impulse (toweling off and pulling my clothes back on), I did dry off but just picked up my clothes and walked back to my cabin through crowds and torchlight, naked and still very hard and vertical. I ended up enjoying the feeling of just being what I was...a young man with a body I like, and having enjoyed being touched sexually: just me with none of the pretensions or barriers that come with clothes. No one seemed offended by it.
I ended up going naked in sessions and in the pool for a couple of hours the next day (though without being hard), and I enjoyed the openness and comfort that comes with that.
I wish I could go to nudist resorts frequently, but there are none in my area. My partner has told me that she would like us to vacation at one later this year, though, so we're looking forward to that.