The only reservation Stash had was the whopper of a dick that Slade has. Stash reserved the right to say, "It ain't gonna fit!" Which thankfully, it did.
I think that is the challenge of a giant cock. You may think to yourself, "I don't think I could take that!" Then, in the heat of the moment, the challenge is thrown down and the next thing ya know, your splitting yourself in two!
That's pretty much what happened with Stash. After sucking on it, he was memserized by its size, and was determined to sit on it...