Tumblr provides great latitude in posting sexually explicit content, but one thing that has always been forbidden is directly uploading sexually explicit video.
Just to be clear, directly uploading is when you select “Upload a video” (on the desktop site) or “Choose a video” (on mobile) when making a video post.
I think the best alternative for posting such a video is putting a teaser image (maybe a GIF) and then linking to the actual the video (uploaded elsewhere, of course) in the post. Here’s a good example. (You can also embed the video, but that can be tough for some sites.)
The rule is part of the Community Guidelines:
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).