Theory: Epstein learnt about 4chan after meeting moot, checked the website a little bit, and then forgot about it.
During the short time he spent on 4chan, he naturally gravitated towards porn content, and he ended up sharing with his friends about /u/ and sfm futanari fnf porn not because he liked these subject in particular, but because it's the first porn he encountered on 4chan, and they left a strong impression on him because of their weirdness (from the point of view of a non-nerd).