Remember when that guy in Part 4 asked Rohan to "draw more girls with big boobs" in his manga? I guess Araki received that in real life, and said "well, I can make Daiya!"
While sound weird that Josuke (19) is getting up with Daiya (16), the "Romeo and Juliet Law" exists, so, so far, in the USA, that's kinda legal. I dunno about other places in the world tho.
I can't say I expected this to reach these levels of horny but I'm not mad, I liked Daiyas design when I first saw it and it looks like araki is on the same wavelength