Just like any other emotion arousal is a response to a thought. It’s a wave of experience that’s washing over you. That part you can control but it’s less important here. The important part is the thought triggering the arousal. When you are in a situation where you think you can experience sexual outcomes (even if just fantasy) you’re likely to trigger your arousal response and pop a boner. These thoughts can also be tied to set and setting. You probably trained yourself not to have erections around family or in places with social stigma around it like most public places. So you have to recognize that you’re having a thought of sexualization, then you can reframe the setting as an inappropriate one for sexual arousal.

In the moment you can respond with deep breathing meditation if that helps. Focus on your breathing and think about the thought and (paradoxically) feel the feelings the thought brings. You’re doing two things. 1. You’re changing your learned response by doing something other than sexualizing a random stranger or sand (which ironically, being silica, is technically rock hard). 2. You’re learning how to recognize the thought, emotion and mechanics of it’s development to better manipulate it.

It’s not like it’s quick and easy, it can take a while and some effort, but not being a pest nor feeling guilty about contributing to women’s daily difficulty is imo worth it. Not only does this make you a better more approachable person in public but learning about your own arousal can make you last longer (if that’s an issue) and have better, more intense and rewarding sex/pleasure. Win win imo!