Not particularly my area of expertise, but yes you’re right, fundamentally the non-consent is a fantasy.
The way it works is you set up an understanding that the dom can do what he wants with you – it’s usually forced sex so playing into rape fantasies. In most cases, sane people still have the whole experience protected by agreed safewords so that you can stop it if you want to.
Some would do it without safewords in place so it feels more real but just try defending that one in court if it all goes horribly wrong – ‘But your Honour, she said it was okay to rape her…’