Well, this is a tricky issue to deal with.
Is watching porn in public properly viewed as harassment?
I am sympathetic to the feminist view expressed here that it virtually is, yet at the same time, it seems to me that a nation that tolerated the page 3 topless model in its national daily press for so long only has itself to blame.
But yes, lines have to (or should) be drawn somewhere, for the sake of civil society, and moving up to watching sex on public transport, within proximity of any other passenger, does deserve a special offence of its own, as a form of public nuisance, I reckon. Perhaps the first step ought to be the right to require them to leave the public space, but if that fails, the back up of potential prosecution is warranted. I think.