You could argue that there are a number of similarities.
First of all, both pornography and prostitution involve people engaging in sexual acts for pay. In that way, the actions being done by the people are similar.
Second, both pornography and prostitution are meant to cater to what some would call the illicit sexual desires of those who consume it. To some people's way of thinking both watching pornography and using a prostitute are violations of sexual morality. In this way, the two are similar from the point of view of those who are "consuming" them.
Finally, there are those who would argue that both prostitution and pornography tend to break down family ties. In both cases, there is a departure from the idea that sexual interest and sexual relations should be kept between spouses only. In this way, the two are similar in their impact on society.