I would imagine that most men don't have a huge range between enough drugs to do lassie and permanent erectile failure.
To get a combination of male genitals and animal whole that can be called sex, I'm guessing that we'd have to go into synthetics territory.
The problem is that we'd definitely have to cross out any drugs like viagra or even rohypnol as they would skew the results of course.
Judging by Charlie in IASIP, with his abortion-surviving and teen-fiance-abstaining characteristics, I'm suggesting Elmer's as the control drug for a negative result.
Also, I'm pretty sure a raccoon would die of internal injuries if you tried to fuck it. They aren't much bigger than a cat.
actually my friend's uncle heard of a guy in his village getting stuck in a chicken, which raccoons of course can eat. I haven't asked my biology teacher yet but I'm pretty sure that if you can eat an animal, you can survive anything that it can survive having sex with. might get stuck tho.