When you're arguing that Tumblr was good because it exposed underage users to abnormal pornography, maybe you need to take a step back and ask yourself how you got to this point.
The distinguishing characteristic of Tumblr porn was vast collections of short (typically 1-2 second) clips depicting extremely rough oral, vaginal, and anal penetration (rough even by porn standards) and abuse porn (violent and degrading treatment of women).
No one in their right mind thinks that children (or adults for that matter) obtaining their sexual socialization from unfiltered internet pornography is a good thing that should be encouraged. The author claims (without proof) that minors are being victimized because they don't have convenient access to an internet pornography free-for-all through household name web companies (Tumblr/Google/Facebook/Twitter/etc). Meanwhile, back in reality, doctors are treating an increasing number of young, frequently underage, girls with incontinence and internal injuries sustained from attempting to emulate what they've seen in porn with their sexual partners.