Say you are single and you win some weird contest, 'fuck your dream pornstar'.
You win and when you meet her before the event, she very politely admits to you that she is not interested in having sex with you, but professionally she has agreed to have sex with the contest winner. However, she assures you that she will do such a great job that you would never know she was uninterested in it by her 'performance'.
Would you still have sex with her?
I don't know about this one. If you fuck her anyway, she'll always remember you as that asshole. But if you give her the chance to pass on the sex, she may change her mind and reward you for your generosity. Then again she may think you're a pussy that probably can't get it up, that's why you backed out. I don't know, I'd probably have to leave it up to her.