There is no @rule stating that anyone must FTB black when asked. And if you read 'help FTB' you will see that the other party does not have to accept a request to FTB and that FTB " must be agreed upon by parties using the local OOC chat" or FTB does not happen.
If I ask to FTB and the other party does not want to FTB, then no FTB occurs. I can choose to RP it away how I want by ICly ending the encounter for example (when it is a sexual scenario) or by 'passing out from the pain' or some such in a torture scenario. But no FTB takes place.
There are, however, rules that state that you can ask for "physical sexual roleplay that is being directed at or requested from your character" to stop via local OOC and that the other player must stop it but this is not the same as FTB. This is making the RP stop and not happen. Not OOCly agreeing it ICly happened without posing it all through.
This is to ensure that, by and large, the only things that ICly happen are the things that ICly happen with sexual scenes being the sole exception.
I just thought I should mention this here due to the what I have read so far in other posts. If I am wrong, I look forward to being corrected. If not, I hope that my bringing this up will help us all get on the same page.