What is the effect of watching porno?

Sexual arousal: Is the goal of the film makers. How it affects you can be very different from one person to the next, just like any other movie.

Sexuality: This may best be asked in other forums. But in general watching porn is a common form of sexuality in many cultures around the world. In general there would be no health or medical reason to even comment on this. The only caveat to that would be obsession or addiction leading to loss of relationships and other forms of intimacy with a real partner.