To be fair, unless that sea of pornography includes pornography involving minors, it isn’t all that relevant. But although I’m pretty disturbed by the original photo and find it inappropriate, there is definitely much worse on Tumblr.
There’s a difference between a duck drawn on a baby. A minor. And two obvious adults doing it in a Gif. The image of the baby didn’t bug me but if the Tumblr staff want it gone, why complain. They have a perfectly good reason.