From their community guidelines...>Unmarked sensitive content. Tumblr is common ground for millions of people from a wide variety of locations, cultures, and backgrounds. Some of them are okay with seeing sensitive content on their dashboard, and some aren't, and that’s fine—that's what Safe Mode is for. While we attempt to automatically categorize content that might not be suitable for all viewers, you can help us respect the choices of people in the community by pre-emptively marking any sensitive material you post.
If you're thinking about posting something that contains sensitive content, please flip the "Mark as sensitive" switch before publishing it (you'll see that switch in the post form). Some examples of sensitive content might include nudity in an artistic, educational, or photojournalistic context.
If you regularly post sexually explicit material, we ask that you mark your Tumblr as "Explicit" (which you can do on your blog's settings page).
Marking your posts or your blog won't prevent you or the people who look at your posts from using any of Tumblr's features—it just helps other Tumblr users avoid seeing content they might not be comfortable with.
Could also be this:>Uploading Sexually Explicit Video. You can embed anything in a Tumblr post as long as it's lawful and follows our other guidelines, but please don't use Tumblr's Upload Video feature to upload sexually explicit video. We're not in the business of hosting adult-oriented videos (and it's fucking expensive).
Strange since there are plenty of amateur video porn blogs that host video on there. Maybe just don't upload the video directly there?