(Fifty Shades is an abomination to BDSM and must be destroyed.)
I'd have to go with Elenor Roosevelt gettin' pounded by an equine mammal. Though, I'd be concerned about the health aspects that come with the danger of taking that large of a penetrative phallas. People have died from that, dudes.
Dying from a heart attack caused by a panic attack, or die because you were that 1% of people who had a really bad side effect from a medicine you were taking.