Hm... I did not see that coming at the end. @Redbone: Don't forget she's the one willing in the first place in this manga. I guess this is what she decided to do after the events of a particular lightnovel volume. xD
========================================================================== SPOIL IN THE END : be careful if you didn't read the last LN about Nadeko. ==========================================================================
Why ryoma does Nadeko with some random characters... Araragi is good. Maybe to show the rage of Nadeko. (good dôjinshi either way)
Edit : well as ever you can't say you don't like something, otherwise you'll get [Vote -]. Anyways if someone can be kind and add in his comment the spoiler tag it'll be nice.
exactly WHO is being ntr'd here, Koyomi never had any sexual interest in her(hell in the anime it seemed he had more interest in his sisters then Nadeko).