Most likely because tumblr kids have daddy issues and anonymous being... well, it's /b/ what can we say?
When 4chan's 'raid' of Tumblr resulted in the realisation that their sexual tension would be best resolved in angrysex fanart and there really is no escaping Rule 34, 4chumblr was born.
Also known as tum/b/lr.
“Come now, Channy,” said Tumblr “we aren’t really all that different.” It paused for a moment and looked down at its shoes. Upon looking up again to meet 4Chan’s gaze, its eyes were glassy and its pupils appeared dilated. “Some of us,” breathed Tumblr “even admire your outrageous sense of humor, meme taste-making abilities, and abundance of cat macros. If anything, we’re jealous of you.” Before it could form any kind of response, 4Chan’s mouth was seized up by Tumblr’s in a passionate crushing of lips. The younger site kissed with the kind of longing born from buying into the sentiments of too many Bright Eyes songs. With horror, 4Chan found itself reciprocating, even fisting its hands in Tumblr’s hair. When the two finally broke apart, the younger site whispered, “Come on baby, let’s find a room. I wanna /b/one you all night long,” and for the first time, 4Chan found itself lacking the definition of air.
-4chumblr from the internet
Already a wide spread of near-porn has been made(and, according to Rule 35, actual porn is on its way), with Tumblr being a hipster girl, and 4chan a green Anon.