No, I don't think it's OK to watch porn in public. You don't watch porn for entertainment like you would watch a Hollywood movie.
You watch porn to get a turn on and to get your business done. I really don't want others to do that in a public place, especially on a public transport where I might not be able to avoid watching them.
Obviously I am talking about real porn and not about some semi erotic flick where some american loses it because you could almost see the tip of a nipple.