Bob,
What do they have to prove?

The Talmudic arguments regarding the issue have already taken place. Take a look at the tractate of Sanhedrin 54A:

הבא על הזכור ועל הבהמה והאשה המביאה את הבהמה בסקילה

Loosely translated: A male who has intercourse with a male or an animal and a woman who has intercourse with an animal are liable (death) by stoning.

This is brought down as law by Maimonides (Laws of Forbidden Relations 1:14) . See also Shulchan Aruch Even HaEzer 24.