re-reading carefully i think it's a given that he did in fact have sex with Banba, the Oni princess. Last chapter panel did show Dondo, her sister being knocked out by Fuhra, the Fairy, and that it would imply that this would cause "something" to happen.
The panels go black as in relation to him recollecting what happened, but the panels with the "fuzz" effect are there to remind us that this event took place in the past. As for Fuhra, we also know that she can manipulate the seeds and flora. This would mean that she did implicitly make it so Kazushi would slip & fall to break his legs (probably by causing something to be seeped into the bath from the tree in it and the tree being something she gave to Kazushi to plant) but also to give him nightmares. Remember he stayed in the bath 1 hour after he had sex with Banba.
What's scary is that he was explicitly told NOT to have sex with anyone. This definitely has the "beginning of the end" vibes for storytelling. Time will tell if this is true.