When you see a man looking at porn in public (and yes, I know it’s not all men but a minority of them), aside from laughing, or feeling scared, or uncomfortable, you might also experience a sense of awe. Imagine, just imagine, having such a sense of ownership of and entitlement to public space that your need to watch a naked woman being penetrated outweighs the discomfort of the women (and children) in the physical environment around you catching you watching it. The lack of respect is staggering.
Personally, I see it as a form of harassment. Most people would say that doing it is socially unacceptable and I hope that other passengers would stand up for the person who chooses to object. It is much easier being brave and assertive when you know other people have got your back.
But it is important also to change the underlying culture, and that means starting young. Not just in the classroom, but at home as well. Too many parents are squeamish about discussing porn. They need to get over this, and start talking about it now, before watching porn becomes even more normalised.