Chastity belt!
Consensual kink, established safe word, no rape or forced sexual relationship. It's, aside from the lack of kink negotiation, surprisingly realistic as far as the genre allows it, and the uke actually and genuinely enjoys himself. Gets a bit vanilla overtime though. Typical "I love you now, so I don't need kink anymore" crap. Also with the hints at the ukes past relationship, I really hope the author doesn't pull a 50 shades with this (I got abused so now I have this fetish).
Slight undertones of a D/s relationship but all in all vanilla.
Zettai, Fukujuu Meirei