Generally, when people claim something is harmless, they don't mean that it's "absolutely harmless". Playing videogames is harmful if you do it to the exclusion of eating, sleeping, and excreting, but one would not normally say that videogames are harmful based on them being harmful under such conditions. It is entirely possible to claim that porn is harmful, and that masturbation under similar circumstances (such as masturbating to mental images of people) is also harmful, while still consistently insisting that masturbation is harmless.