"In talking to the subjects, researchers discovered that high exposure to pornography videos apparently resulted in lower responsivity and an increased need for more extreme, specialized or “k+++y” material to become aroused."[1]
This effect can be clearly seen in that pornography websites promote this extreme, vile and obscure content, such as incest, exhibitionism, and even depictions of non-consensual interaction and physical abuse.[2] Obviously, these matters have no place in a healthy relationship, and it's pretty basic psychology that regular consumption of this content causes the normalization of such practices, especially in impressionable teenagers whom do not yet have legitimate experience in healthy, normal relationships.
A majority of adults watches pornography.[3] And we're dealing with a massive loneliness epidemic under younger generations, together with a significant rise in "hook-up culture" over forming serious relationships. Coincidence?
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5039517/
[2] Just go to one of those websites. I'm not going to do that, neither am I going to link to that here.
[3] https://www.statista.com/statistics/1402222/us-adults-pornog...
I'm not sure why legislators aren't trying to address [2], which is the real problem as far as I can tell.
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Regarding [1], the study itself mentions that stopping watching porn reverses the effect. In layman's terms: watch enough of it and the novelty wears off, but the sexual drive returns. Hardly a harm, it's what happens with every human activity.
[2] makes the big logic jump of assuming that someone who watches kinky porn fails to separate between fantasy and reality. It is the same line of reasoning as the disproven "videogames cause violence" paradigm and it is pushed by the same sort of people (personal hypothesis: they might be projecting). This could ironically point to a problem limited to at least some individuals failing to differentiate the two, but studies find that at the population level, a higher availability of porn correlates with lower rates of sexual assault. My personal reading is that it provides a safe outlet for sexual frustration and moderate desensitization reduces the chance that someone will, so to speak, get aroused over an exposed ankle.
On [3]... you're linking to a single data point, not a series nor a correlation; additionally, even if the correlation actually existed held, people's propension to form stable relationships is a preference, not a harm. It is also not related to minors, and it is not something that the state has any business sanctioning, much less with incarceration.