Boners are largely age dependent. I remember in high school I got random boners all the time and anything could trigger them. Now, at 40, I have way more control plus novelty matures and wears off IMO.

You should have said hello or tried – YOLO. Those opportunities dwindle with age for the most part.

Try to step out of your own perspective in a mental abstracted exercise and ask yourself, ‘If that was me is there any chance I would be touching someone like this and not notice?’ Then, ‘What is the least amount of meaning I would attribute to such a gesture? – (now conservatively divide that by /10).’ Finally, ‘If I were in this person’s state of mind, how would I perceive a stranger talking to me?’ IMO, when with a friend, the person is going to be on the high ground and feeling secure and safe from embarrassment. If you directly address the touching, you’re going to be at a disadvantage and get shut down. However, if you can find a way into making conversation, the touching was like an invitation to do so. By not addressing it directly you are using it as a step up into exploring more. The hard part is just the small talk. Many people are just more tactile, but in our present world, touching strangers (at least in the USA) is very rare.