This is the exact verbiage from the community guidelines:
Sexually Explicit Material. Visual depictions of sexually explicit acts (or content with an overt focus on genitalia) are not allowed on Tumblr. That includes pictures, videos, GIFs, drawings, CGI, or anything similar. Historically significant art that you may find in a mainstream museum and which depicts sex acts—such as from India’s Śuṅga Empire—are now allowed on Tumblr with proper labeling.
Nudity and other kinds of adult material are generally welcome. We’re not here to judge your art, we just ask that you add a Community Label to your mature content so that people can choose to filter it out of their Dashboard if they prefer.
You have the option to add a community label when making a new post, reblogging a post, or editing an existing post. Depending on your content, you can label it as generally mature or choose a specific category such as “Sexual Themes” if your post contains sexually suggestive subject matter.
Blogs which have a focus on mature content may not be eligible for certain Tumblr features, including monetization options. We need to consider the policies of our partners in the payments space, so the rules there are a bit different.
For more information about this guideline, and how to appeal decisions about sexually explicit material, check out our Help Center article.
Yeah because they don't have to spend any money on R&D nor do they spend money contributing to the open source project :)