I appreciate that this is one of those rare futa doujins where the plot isn't basically "Holy fuck, dicks are AMAZING!" But on the other hand the presence of the dick is a bit baffling as there's no real reason for Rin to have one in the first place. You could do the exact same scene without it, and nobody would realize. So...really, why bother?
I don't hate it, but I'm not going to re-read it. Put me down for a "shrug and walk away" reaction.
It's probably because sex is much more pleasurable and meaningful with a dick.