Everything has some level of harm, the issue here is comparing relative vs absolute harm. Pointing out a net positive doesn’t invalidate other harms, but to find the net effect it must be considered.

Also, there’s significant questions around porn and early sexual activity with evidence in both directions. Unsurprisingly, people be complicated.

Just as an example the “explosion” of internet porn would if the theory is correct presumably have had some population impacts on when people first had sex but 1997 to 2007 showed zero changes. “The proportion of high school students who were sexually active has remained steady since 1997” https://pmc.ncbi.nlm.nih.gov/articles/PMC3064497/

Obviously being a survey the data is suspect but still a meaningful datapoint here.