In Foster's Home for Imaginary Friends, any child can make an imaginary friend. Eventually it was canonized that teenagers could also create an imaginary friend. When an imaginary friend is created in universe, it enters the universe exactly as the imaginer imagined them.
The Meme is implying that teens are making imaginary friends (who have bodies and a physical form) for carnal, hormone fueled purposes.
Edit: I should specify, this was explaining the meme, and is actually not correct in reference to canon.