Imagine your fictional other feeling insecure. You ask how you may reassure them that you will love them forever and never leave. They suggest they want to keep you permanently tied up; they explain that you agreeing to this would be a promise of love to them, proof that you are now theirs completely and you choose to be unable to ever leave.
They get you beautiful clothes, soft and comfortable, but so restrictive you can hardly move in them. Perhaps they get your body pierced and decorate it with jewellery that links your body parts together/to furniture items to restrain you. You could never escape. You can't take care of yourself anymore. You are completely reliant on them, and you know they will take the best care of you, but they would never ever let you go. Your bonds are like a constant reminder of their embrace.
Sometimes they take you out for a walk, still bound, barely able to shuffle forward; they blindfold you, so you cannot even look at anyone else and can't try to navigate the world without their help even with the limited movement that you are only even allowed for the sake of accompanying them, and gag you, so you can't speak to anyone else; they are your whole world now, so why would you need to? They are so proud of showing off their beloved who made themself so elegantly and willingly helpless for their sake, who gave up everything for them, the whole world and even the use of their own body.
Every day, they praise you so much for giving yourself to them like this. They love you even more now, and they will make sure you feel loved and you never feel the burden of having to do anything for yourself ever again. They are certain everyone must envy them for having you, but no one else can. After all, you can't even interact with anyone else, and you couldn't leave even if you wanted.
Or, if you prefer, imagine keeping your F/O that way. They would willingly agree to this just to convince you they are yours.
