I wear mesh basketball shorts or track pants almost exclusively when I'm not at work. When I switched from briefs to boxers, I had the same reaction. I got hard quite a few times in public, usually from feeling my dick bounce around while I was walking and/or the head rubbing against the fabric. I don't worry if I get semi hard. It feels good so I keep walking and let it extend down my right leg. A full on boner when walking requires that I adjust it, or stop for a minute to keep it from hurting. Only the people looking for dick really see it, and most of the time they are guys. Once I got used to hanging free in boxers, freeballing was virtually the same thing. I always freeball in mesh shorts at the beach. Feels great in the ocean. People see the whole deal when I exit the water. If they want to look, they can look. Anything goes at the beach as long as you're clothed.