I have been online looking at naked female statuary for a friends garden this is true. Off to sleep now but to help explain further here is a deconstruction, kind of, of your material
'I'm a normal bloke I don't rape women... Unless ..you know... they are really fit. Or they are black because they are genetically predisposed to enjoy it.'
You might say the twist in the first part is the old woman being raped. Ask yourself the question is it funny because it does happen, because it doesn't happen or because it isn't normal.
Do it yourself and see where the funny lies what they call the 'target' in comedy. Comedy is very hard work and right here is where you start paying - to paraphrase a fit, black woman!